Type Narrow Exact Strings

Another feature suggestion 😅

It would be useful to be able to type-narrow exact strings.

For example:

---@alias TestTable {test_group_a: {test_field_1: string, test_field_2: string}, test_group_b: {test_field_1: string, test_field_3: string}}

---@type "test_group_a" | "test_group_b"
local test

if test == "test_group_a" then
	---@type TestTable
	local test_table
	test1 = test_table[test].test_field_1
end

When we do if test == "test_group_a" then it would be nice if the type of test narrowed to test_group_a (which can then be used to successfully index test_table. As it is now there is no type narrowing so the inferred type of test1 is unknown when it should be string.

issue 中的 Lua 示例是唯一提供的入口点；使用它来复现 test 和 test1 当前的类型。完成的标准是：与 "test_group_a" 进行相等性检查后，test 被收窄到足以索引 TestTable，并推断 test1 为 string，同时覆盖所示情形。