[lua] Stub EReg for -D lua-vanilla mode

[jdonaldson]

Note for simn, I'm gradually chipping away at some of the leftover issues from Lua. And jotting down a few points where I hit some foot guns. I'm using AI to help generate stub files and to write tests. It's all things I wanted to do before but just couldn't find the motivation/time. I can finish off the rest of the lua-vanilla issues, but wanted to give you a chance to see the basic intent before I added a bunch of stuff.

Summary

• EReg now conditionally compiles: stub implementation for -D lua-vanilla, real implementation otherwise
• Stub throws NotImplementedException at runtime with a clear error message
• Partial fix for [lua] -D lua-vanilla is ignored #12063 - removes the lrexlib-pcre2 dependency when using lua-vanilla

Background

The #error directive approach in _std override files triggers eager file scanning, causing compilation failures even when the type isn't used. This PR uses conditional compilation with a runtime stub instead.

Remaining work for #12063

Other modules still need the same treatment:

• Sys.hx (luv)
• sys/FileSystem.hx (luv)
• sys/io/Process.hx (luv)
• sys/net/*.hx (luasocket)
• sys/ssl/Socket.hx (luasec)
• haxe/iterators/StringIterator.hx (lua-utf8)

Test plan

• Verify EReg compiles without lrexlib-pcre2 when using -D lua-vanilla
• Verify EReg throws NotImplementedException at runtime in vanilla mode
• Verify normal EReg functionality still works without -D lua-vanilla

[Simn]

Use -D lua-vanilla=false or install lrexlib-pcre2.

This sounds like installing lrexlib-pcre2 would make it work, which I don't think is the case if we're compiling with the stub-implementation.

[jdonaldson]

Yeah, this feels awkward to me too. And it's why I only showed one class and stopped short of stubbing the rest out.

The stub implementation is only included/compiled with -D lua-vanilla, which will only get included if the EReg class gets imported somewhere/somehow in a given target. If one is compiling with -D lua-vanilla, it means one is in a situation where one can't include any 3rd party libraries. This often the case when writing Lua mods, plugins, etc.

LMK if I'm missing something here, it's been a loooong time since I thought about this, and the only reason I'm taking another swing is because I had some leftover Claude code credits. :)

also, happy new year! Warm wishes.