content: Handle keyword highlighting in code blocks

[rajveermalviya]

Take 2 of #1707

Fixes: #1695

Screenshots when commits based on top of #1694:

Flutter	Web

[gnprice]

Thanks!

I see some timings, including for this PR, in #1707 (comment), which are very helpful. Can you repeat those timings atop main, without this PR's changes?

Those show about 2ms for parsing a long code block. That's tolerable if we can't easily do better; but it's nontrivial, being a sizable fraction of a frame interval. So I'm curious how much of that is a regression due to what's added in this PR, and how much of it is already in our current logic.

[rajveermalviya]

Here are the timings for main and this PR when running the release version of the app on my Android device running Lineage OS 22.2 (Android 15).

Current main

I/flutter ( 7160): parseCodeBlock took 1875us
I/flutter ( 7160): CodeBlock.build took 168us
I/flutter ( 7160): parseCodeBlock took 2838us
I/flutter ( 7160): CodeBlock.build took 383us
I/flutter ( 7160): parseCodeBlock took 1519us
I/flutter ( 7160): CodeBlock.build took 287us
I/flutter ( 7160): parseCodeBlock took 2497us
I/flutter ( 7160): CodeBlock.build took 214us
I/flutter ( 7160): parseCodeBlock took 1868us
I/flutter ( 7160): CodeBlock.build took 183us
I/flutter ( 7160): parseCodeBlock took 1793us
I/flutter ( 7160): CodeBlock.build took 200us
I/flutter ( 7160): parseCodeBlock took 2135us
I/flutter ( 7160): CodeBlock.build took 243us
I/flutter ( 7160): parseCodeBlock took 2276us
I/flutter ( 7160): CodeBlock.build took 212us
I/flutter ( 7160): parseCodeBlock took 1556us
I/flutter ( 7160): CodeBlock.build took 250us
I/flutter ( 7160): parseCodeBlock took 2206us
I/flutter ( 7160): CodeBlock.build took 386us

Tree (this PR)

I/flutter (25604): parseCodeBlock took 1542us
I/flutter (25604): CodeBlock.build took 192us
I/flutter (25604): parseCodeBlock took 1401us
I/flutter (25604): CodeBlock.build took 228us
I/flutter (25604): parseCodeBlock took 1884us
I/flutter (25604): CodeBlock.build took 207us
I/flutter (25604): parseCodeBlock took 1772us
I/flutter (25604): CodeBlock.build took 211us
I/flutter (25604): parseCodeBlock took 1993us
I/flutter (25604): CodeBlock.build took 235us
I/flutter (25604): parseCodeBlock took 2388us
I/flutter (25604): CodeBlock.build took 362us
I/flutter (25604): parseCodeBlock took 1716us
I/flutter (25604): CodeBlock.build took 219us
I/flutter (25604): parseCodeBlock took 2008us
I/flutter (25604): CodeBlock.build took 366us
I/flutter (25604): parseCodeBlock took 2239us
I/flutter (25604): CodeBlock.build took 373us
I/flutter (25604): parseCodeBlock took 1720us
I/flutter (25604): CodeBlock.build took 262us

So looks like that the parsing step is faster with this PR, 1.4/1.8/2.3ms (min/avg/max) where on main it takes 1.5/2.0/2.8ms. While rendering step, this PR takes 192/265/373us and main takes 168/252/386us, so main is slightly faster there.
Overall when combining both timings, this PR takes 1.6/2.1/2.7ms and main takes 1.8/2.3/3.2ms.

[gnprice]

Great, thanks for checking that. So there's potentially room for improvement in our parse times for code blocks (as there might be for other content too) — but this PR moves that in the right direction.

(And I think it's not currently a priority to try to optimize that parse time any further — it's tolerable that a giant message might take a couple of ms when it first appears.)