Update to Lexilla/Scintilla 5.x and Scintillua 6.x

[AHSauge]

This is a fairly bulky merge request, but this does upgrade to Lexilla/Scintilla 5.x and Scintillua 6.x

Some things to note

1. Scintilla is sadly no longer found in git (only Mercurial), so I very reluctantly had to drop all that code in. Everything under dep/scintilla/scintilla should be an unmodified copy from Mercurial.
2. Scintilla (I think; been a while since I worked on this) dropped theme support and relies on the application doing it for them. I didn't bother figuring this out myself, but my friend Claude was happy to do it instead 😛 I've reviewed this as best as I can, but there are some blocks with NOTE: This is AI generated code where there's bit more than a few lines changed. I don't see anything wrong in the code, but worth being aware of. In any case, this is the riskiest change here in terms of visual and behavioural changes.
3. The diff context menu is now fully created by src/editor/TextEditor.cpp. I've kept the context menu as-is, but there are some entries that quite obviously could be dropped.

@Murmele This is draft solely because I'm still testing this, but I would still very much appreciate reviewing. I don't anticipate large changes at this stage, but only minor tweaks to make sure the theme and style isn't broken.

Fixes #935

[AHSauge]

Ugh! Gotta love different include path strategies between *nix and Windows 😫 Anyway, finally builds on Windows. Look wise, it seems pretty similar on Linux and Windows