A Raku lexer was added to upstream Scintilla some time ago: https://sourceforge.net/p/scintilla/feature-requests/1328/
You'll notice that Geany includes only a curated subset of upstream lexers: https://github.com/geany/geany/tree/master/scintilla/lexilla/lexers
Getting Raku into Geany would require a PR similar to these recent examples:
- https://github.com/geany/geany/pull/2584 - https://github.com/geany/geany/pull/3012
The main difference here is that the lexer module [is already written](https://github.com/ScintillaOrg/lexilla/blob/efd741310e9bd870811ec55940d05ff...); the main job would be creating a tags parser and a file definition for Raku.