[lua] hx-lua-simdjson breaks -D lua-vanilla

[shakesoda]

Per my comment on #9885: when using -D lua-vanilla (binary/external deps are a no-go), the flag is not respected as it was previously, and I get failed requires.

There should be a fallback to the generic parser when this flag is in use. I've worked around the issue by copying the generic parser out of std for the time being.

[jdonaldson]

Thanks for your patience. I do agree this is problematic and I need to support "vanilla" development more robustly. I need to think through the design choices in more detail, I'm not sure if the "vanilla" flag is a good design pattern.

[inklit]

any update on this?