Remove obsolete highlighting code

[mgarstenauer]

I suggest removing the orginal (obsolete) syntax highlighting code in the AvaloniaEdit.

Background info:
The original AvalonEdit implementation used its own highlighting engine. AvaloniaEdit instead uses TextMate.
The original highlighting code is still included but not used anywhere. See folder src/AvaloniaEdit/Highlighting.

Pros:

• AvaloniaEdit.dll shrinks by 25%. 662 KB → 502 KB, measured on Win11, .NET 8, Release build.
• Maintain/support only the modern highlighting implementation. Remove obsolete/unmaintained/unsupported code path.

Cons:

• Breaking change.
• A 1:1 app port from WPF AvalonEdit to AvaloniaEdit will not have syntax highlighting. It is easy to switch to TextMate, but it will not be possible to use custom AvalonEdit XML syntax highlighting definition files (.xshd files).
• The HtmlClipboard and RichText classes will have to be removed. Most users probably have never heard of these features in AvaloniaEdit. ;)

Let me know what you think! I can provide the PR – the change is fairly trivial.
If not now then maybe we should consider this for v12?

[goswinr]

I still use it. How about putting it in a separate nuget package?

[mgarstenauer]

@goswinr Good to know. Is this for legacy reasons (e.g. coming from AvalonEdit)? Or is there an issue (performance, missing feature) with the TextMate variant?

I like your suggestion. Moving it to a separate package is a clean solution.

[goswinr]

Just legacy, I am porting my F# script editor from AvalonEdit. I customized my xshd a lot. Haven't tried Textmate yet.