fix: use return_overload annotations in std libs, to fix overload return type inference

[EfveZombie]

Summary

Replace ---@return type? patterns with ---@return_overload annotations in standard library definitions, to fix overload return type inference for functions that return either a success value or nil, error_message.

Changed files

• global.lua: load, loadstring, loadfile
• io.lua: io.open, io.write, file:close, file:flush, file:seek, file:write
• package.lua: package.searchpath
• utf8.lua: utf8.len

Motivation

The previous ---@return type? annotations merged all return values into a single nullable type, which caused the type checker to lose overload information. By using ---@return_overload, each return variant is expressed as a distinct overload, allowing the analyzer to correctly narrow types based on success/failure branches.

[gemini-code-assist]

Code Review

This pull request updates the Lua standard library annotations by replacing @return tags with @return_overload in global.lua, io.lua, package.lua, and utf8.lua. A review comment points out that package.searchpath was incorrectly modified to return a file object instead of a string, which is a regression from the previous state.