Nested Table Type Narrowing

It would be super nice to be able to get field suggestions when narrowing an indice of a nested table.

Currently this:

---@class TestClass
---@field test_group_a TestGroupA
---@field test_group_b TestGroupB

---@class TestGroupA : TestGroupCommon
---@field test_field_1 string
---@field test_field_2 string

---@class TestGroupB : TestGroupCommon
---@field test_field_1 string
---@field test_field_3 string

---@class TestGroupCommon
---@field test_field_common string

---@type TestClass
local test_class

---@type 'test_group_a' | 'test_group_b'
local test_group

local test1 = test_class[test_group].test_field_1


---------------------------------------

---@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 TestTable
local test_table

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

local test2 = test_table[test_table_group].test_field_1

The inferred type for test1 and test2 are both unknown. It would be nice if for test1 and test2 the inferred type would be string (as test_field_1 is a string for both, if test_field_1 and test_field_2 differed the inferred the type should be test_field_1_type | test_field_2_type). It would also be nice if on local test1 = test_class[test_group]. there were field suggestions for test_field_1 and test_field_common which are on both TestGroupA and TestGroupB.

Note: to add this for classes would also require doing: https://github.com/LuaLS/lua-language-server/issues/2217

先重現 issue 中的兩個 Lua 範例，並檢查處理索引存取、型別縮小和欄位補全的 language-server 路徑。完成的標準是：兩個範例都能為 test_field_1 推斷出 string，並提供可能的巢狀表格型別共有的欄位，同時考量相關的類別縮小問題 2217。