[Feature request] Implicit casts for common conversions

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, 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.

まず、issue に記載されている既存の implicit-cast と type-inference の挙動を追跡します。特に、後続の代入が以前の変数宣言にどのような影響を与えるかを確認します。意図する推論ルールを定義し、コマンドライン解析の例に照らして検証します。合意された挙動が定まり、その結果推論される型をカバーするテストがあれば完了です。