[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.

首先追踪 issue 中描述的现有 implicit-cast 和 type-inference 行为，尤其要关注后续赋值如何影响较早的变量声明。定义预期的推断规则，并根据命令行解析示例对其进行验证；完成的标准是就行为达成一致，并有测试覆盖最终推断出的类型。