Start of Lua support

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diff --git a/src/theming/lua-lib/expect.lua b/src/theming/lua-lib/expect.lua
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+-- SPDX-FileCopyrightText: 2019 The CC: Tweaked Developers
+--
+-- SPDX-License-Identifier: MPL-2.0
+
+--[[- The [`cc.expect`] library provides helper functions for verifying that
+function arguments are well-formed and of the correct type.
+
+@module cc.expect
+@since 1.84.0
+@changed 1.96.0 The module can now be called directly as a function, which wraps around `expect.expect`.
+@usage Define a basic function and check it has the correct arguments.
+
+    local expect = require "cc.expect"
+    local expect, field = expect.expect, expect.field
+
+    local function add_person(name, info)
+        expect(1, name, "string")
+        expect(2, info, "table", "nil")
+
+        if info then
+            print("Got age=", field(info, "age", "number"))
+            print("Got gender=", field(info, "gender", "string", "nil"))
+        end
+    end
+
+    add_person("Anastazja") -- `info' is optional
+    add_person("Kion", { age = 23 }) -- `gender' is optional
+    add_person("Caoimhin", { age = 23, gender = true }) -- error!
+]]
+
+local native_select, native_type = select, type
+
+local function get_type_names(...)
+    local types = table.pack(...)
+    for i = types.n, 1, -1 do
+        if types[i] == "nil" then table.remove(types, i) end
+    end
+
+    if #types <= 1 then
+        return tostring(...)
+    else
+        return table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
+    end
+end
+
+
+local function get_display_type(value, t)
+    -- Lua is somewhat inconsistent in whether it obeys __name just for values which
+    -- have a per-instance metatable (so tables/userdata) or for everything. We follow
+    -- Cobalt and only read the metatable for tables/userdata.
+    if t ~= "table" and t ~= "userdata" then return t end
+
+    local metatable = debug.getmetatable(value)
+    if not metatable then return t end
+
+    local name = rawget(metatable, "__name")
+    if type(name) == "string" then return name else return t end
+end
+
+--- Expect an argument to have a specific type.
+--
+-- @tparam number index The 1-based argument index.
+-- @param value The argument's value.
+-- @tparam string ... The allowed types of the argument.
+-- @return The given `value`.
+-- @throws If the value is not one of the allowed types.
+local function expect(index, value, ...)
+    local t = native_type(value)
+    for i = 1, native_select("#", ...) do
+        if t == native_select(i, ...) then return value end
+    end
+
+    -- If we can determine the function name with a high level of confidence, try to include it.
+    local name
+    local ok, info = pcall(debug.getinfo, 3, "nS")
+    if ok and info.name and info.name ~= "" and info.what ~= "C" then name = info.name end
+
+    t = get_display_type(value, t)
+
+    local type_names = get_type_names(...)
+    if name then
+        error(("bad argument #%d to '%s' (%s expected, got %s)"):format(index, name, type_names, t), 3)
+    else
+        error(("bad argument #%d (%s expected, got %s)"):format(index, type_names, t), 3)
+    end
+end
+
+--- Expect an field to have a specific type.
+--
+-- @tparam table tbl The table to index.
+-- @tparam string index The field name to check.
+-- @tparam string ... The allowed types of the argument.
+-- @return The contents of the given field.
+-- @throws If the field is not one of the allowed types.
+local function field(tbl, index, ...)
+    expect(1, tbl, "table")
+    expect(2, index, "string")
+
+    local value = tbl[index]
+    local t = native_type(value)
+    for i = 1, native_select("#", ...) do
+        if t == native_select(i, ...) then return value end
+    end
+
+    t = get_display_type(value, t)
+
+    if value == nil then
+        error(("field '%s' missing from table"):format(index), 3)
+    else
+        error(("bad field '%s' (%s expected, got %s)"):format(index, get_type_names(...), t), 3)
+    end
+end
+
+local function is_nan(num)
+  return num ~= num
+end
+
+--- Expect a number to be within a specific range.
+--
+-- @tparam number num The value to check.
+-- @tparam number min The minimum value, if nil then `-math.huge` is used.
+-- @tparam number max The maximum value, if nil then `math.huge` is used.
+-- @return The given `value`.
+-- @throws If the value is outside of the allowed range.
+-- @since 1.96.0
+local function range(num, min, max)
+  expect(1, num, "number")
+  min = expect(2, min, "number", "nil") or -math.huge
+  max = expect(3, max, "number", "nil") or math.huge
+  if min > max then
+      error("min must be less than or equal to max)", 2)
+  end
+
+  if is_nan(num) or num < min or num > max then
+      error(("number outside of range (expected %s to be within %s and %s)"):format(num, min, max), 3)
+  end
+
+  return num
+end
+
+return setmetatable({
+    expect = expect,
+    field = field,
+    range = range,
+}, { __call = function(_, ...) return expect(...) end })