[Feature request] Implicit casts for common conversions

Nobody has claimed this yet.

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.

Start by tracing the existing implicit-cast and type-inference behavior described in the issue, especially how later assignments affect an earlier variable declaration. Define the intended inference rules and verify them against the command-line parsing example; done requires an agreed behavior and tests covering the resulting inferred type.

Written by the indexing model from the issue text.