Add syntax highlighting support for lines

[patriksvensson]

No description provided.

[LPeter1997]

We have got a project that we would love to use the library for and would like to have some syntax coloring. I also don't mind contributing this.

Having fairly minimal Spectre knowledge, peeking around in the library it looks like the source lines are rendered here. The actual highlighter API could be exposed in the ReportSettings, I do not see any other public API that's already present for the renderer. Something simple could be exposed for the user:

readonly record struct Token(int Start, int Width, string Type);

interface ISourceTokenizer
{
    public IEnumerable<Token> TokenizeLine(string sourceId, int line, string line);
}

So render text could be driven based off of this (or something similar). Color coding could be done by mapping string Types to colors. For a CLI renderer, even just having a fixed set of token types could be fine, in which case the type could be an enum instead.

What kind of API did you imagine for this? Have you planned to pick this issue up yourself, or should we consider contributing this ourselves? If this is the case, do you have any pointers/suggestions for us?