[Feature request] Implicit casts for common conversions

還沒有人認領這個 Issue。

I got some code for parsing command line flags/user inputs that commonly uses patterns like this

```
---@return string ...
local function strsplit(str, sep) end

local a, b, c = strsplit(userinput, ",")
b = tonumber(b) or 0 -- error: This variable is defined as type `string`. Cannot convert its type to `number`
```

The fix is to define b as `string|number` in the declaration, but it's a bit cumbersome, especially if I just want `b` as a number:

```
local a,
---@type number|string
b, c = strsplit(userinput, ",")
```

So it would be nice if common type conversion patterns like this could be recognized automatically and either allow the cast or infer the type as string|number from the beginning.

Edit: See [comment below](https://github.com/LuaLS/lua-language-server/issues/3027#issuecomment-2572687841), the implicit cast is already happening, this is really about type inference based on later assignments. Which may or may not be a good idea.

首先追蹤 issue 中描述的現有 implicit-cast 和 type-inference 行為，尤其要關注後續指派如何影響較早的變數宣告。定義預期的推論規則，並根據命令列解析範例進行驗證；完成的標準是對行為達成共識，並有測試涵蓋最終推論出的型別。

由索引模型根據 Issue 內容生成。