[lua] Fix closure on string method causing runtime error

[jdonaldson]

Summary

Fixes #8068 on the Lua target.

Extracting a method from a string as a closure (var o = {charAt: f.charAt}) crashes at runtime because:

1. Native Lua strings don't have Haxe methods on their metatable. The generator emits _hx_bind(f, f.charAt), but f.charAt resolves to nil through Lua's default string metatable. Fixed by looking up the method from String.prototype instead when the expression is String-typed.

2. _hx_bind crashes on non-table primitives. It calls rawset(o, '_hx__closures', {}) which fails on strings. Fixed by detecting non-table types and returning a fresh bound closure without caching. This covers the Dynamic/untyped case where the generator can't know the type at compile time.

Note: the JS target has the same $bind issue (tries to set hx__closures__ on a string primitive). This PR only addresses Lua.

Test plan

• New regression test Issue8068 — extracts charAt from a string, calls it through an anonymous object
• All Lua unit tests pass (11661 assertions, 0 failures)
• Standalone repro from original issue returns correct result

[Simn]

I generally think this should be fixed in the run-time because whatever problem can occur with properly typed code can also occur when accessing it through Dynamic. I don't know if that's something we can do on lua though, so maybe this is alright. Let's double-check that!