Type Narrow Exact Strings

Nobody has claimed this yet.

Another feature suggestion 😅

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

For example:
```lua
---@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.

The issue's Lua example is the only entry point provided; use it to reproduce the current types of test and test1. Done means an equality check against "test_group_a" narrows test enough to index TestTable and infer test1 as string, including coverage for the shown case.

Written by the indexing model from the issue text.