Add a way to manually update SyntaxHighlighter's highlighting without relying on lines_edited_from()

[mouzedrift]

Describe the project you are working on

I'm implementing a C++ code editor in my app in combination with the LSP to support proper C++ syntax highlighting, but I cannot find a way to implement this asynchronously by subclassing SyntaxHighlighter using the current methods/properties in godot 4.4 mono.

Describe the problem or limitation you are having in your project

The problem I have is that the only way I can get _get_line_syntax_highlighting to be called after calling clear_highlighting_cache or update_cache is by waiting for the lines_edited_from signal. This works if everything runs synchronously, but I'm trying to await the semantic tokens, and this makes the SyntaxHighlighter cache old/cleared data. From what I've observed, TextEdit only seems to update the text colors when you make changes to the text.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

A way to manually trigger _get_line_syntax_highlighting from the subclass to update the syntax highlighting without relying on TextEdit text changes.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Not really sure what the best approach would be, but having a method to make the SyntaxHighlighter manually call _get_line_syntax_highlighting to update the cache could be useful, or perhaps update_cache could do that as well.

If this enhancement will not be used often, can it be worked around with a few lines of script?

A small workaround I found is setting the syntax_highlighter property to null and then reassigning it again, but it makes the text flicker, and that's not very nice to look at.

Is there a reason why this should be core and not an add-on in the asset library?

This seems like a must have.