Upgrade to lexilla and scintilla 5.x

[AHSauge]

I think the last (C++) dependency that's vendored now is lexilla (nice to have for #817), and it would be neat to get rid of that forced vendoring. This is somewhat non-trivial since lexilla typically isn't shipped in distros. However, that could be changed. The other objective here is to shed off scintilla since it's inherently building against GTK. It would seem like the intention with the split in 5.x is to create a clean separation of non-GUI aspects of the library. My understanding is that that is lexilla. With that knowledge, it might not be too bad upgrading, so I'll give it a go.

Indirectly I suspect this also means updating scintillua or changing it's integration. Currently it seems we only extract the lua files from the project, and don't build any of the code. Quite suspicious, but I'll leave that for a separate issue if needed.

Edit 26.01.2026: Packaging is not possible due to lack of ABI stability in the library

Remaining work (up-to-date per 01.02.2026):

• All instances of TODO: Port to Scintila 5.x need to be fixed
• Some tests are segfaulting
• Theme / styling doesn't work. This was dropped by Scintillua, so we need to manage and set this ourselves
• The Lua based lexer / scintillua, isn't successfully created
• Context menu doesn't work. Needs to be created outside Scintilla due to lacking API
• Test that stuff still works

[AHSauge]

This is an incredibly painful upgrade, but WIP is here: https://github.com/AHSauge/Gittyup/tree/feature/lexilla-5.x

It still doesn't compile, but I'm hoping it's a matter of sorting out the annoying case changes between the custom interface class that existed before and Scintilla::ScintillaCall, and the porting the lexer creation to use 5.x workflow.

[AHSauge]

Oh, and shedding off Scintilla is absolutely not an option. The core of Scintilla is GUI-neutral, but is very much required to build the Qt integration. In fact, most of the bindings are usable to us, and while typing this out I'm realising that I went about this the wrong way by using ScintillaEditBase. I should probably run the generator for ScintillaEdit and use that. It might be that we can really ditch the entire custom Scintilla code, which would be absolutely amazing.

[AHSauge]

Seeing as I keep force-pushing updates to my branch, and in case I give up on this (it's a pretty painful upgrade). The current state is that Gittyup starts, but there are numerous issues

• All instances of TODO: Port to Scintila 5.x need to be fixed
• Some tests are segfaulting
• Theme / styling doesn't work
• I'm pretty sure any formatting tests will fail outright. How do we ignore formatting on imported code? 🤔
• The Lua based lexer / scintillua, isn't successfully created
• Context menu doesn't work
• ...and probably a bunch other things

On the upside, this drops 9200+ lines of custom code, mostly duplicate of Scintilla and Scintillua.

[AHSauge]

I'm slowly getting this into a working state. Initial description is updated with a todo list. I'll keep updating it. Generally this doesn't look (lack of theme aside) too bad at this point. Fixing the context menu feels pretty doable, so the biggest hurdle right now is getting theme back in a working state.