Difference in behavior between table literal, dictionary, and class in terms of completion

まだ誰も着手していません。

Maybe more of a feature request than a bug, but I noticed some interesting differences between classes, dictionaries, and table literals in terms of completions.

Using this code as a base:
```lua
---@class TestClass
---@field test_group TestGroup

---@class TestGroup
---@field test_field string

---@alias TestTableDictionary {["test_group"]: {["test_field"]: string}}

---@alias TestTableLiteral {test_group: {test_field: string}}

---@type TestClass
local test_class
local test_group = "test_group"

local test1 = test_class["test_group"].test_field

---@type TestTableDictionary
local test_table_dictionary
local test_group = "test_group"

local test2 = test_table_dictionary[test_group].test_field

---@type TestTableLiteral
local test_table_literal
local test_group = "test_group"

local test3 = test_table_literal[test_group].test_field
```

I would expect the completion for test1, test2, and test3 to be equivalent. However there are some interesting differences.

For test 1 if we do this `local test1 = test_class[test_group].test_field` the type of test1 is unknown and there is no completion suggestions for `test_class[test_group].` Both `test_class["test_group"].test_field` and `test_class.test_group.test_field` work correctly.

For test 2 `test_table_dictionary[test_group].test_field` works correctly. `test_table_dictionary.test_group.test_field` gives the right value but does not show any completion suggestions like the classes do.

For test 3 the table literal is the best of all worlds. You can do ` test_table_literal[test_group].test_field` as well as get completions on test_table_literal.test_group.test_field.

As I am using classes in a lot of places and they are useful as they can be spread across multiple lines and the fields can have longer descriptions it would be awesome to see the ability to do something like `test_class[test_group].test_field` for classes

この issue ではソースファイルやテストが指定されていないため、3 つの Lua の例を使って、class fields、dictionary aliases、table literals に対する language server の completion 処理を追跡することから始めてください。等価なインデックス式とドット式で、期待される型と completion の候補が提供されれば完了です。

索引モデルが issue の本文から書いたものです。