Optimize syntax highlighting, minor web cleanup

[HushBugger]

1. Make syntax highlighting of large files much faster. Especially on Firefox loading https://code.deltarune.wiki/ch4/gml_globalscript_scr_text takes ages and blocks the page while highlighting happens, now it takes less time and doesn't block the page.
   The new code processes the HTML manually, which I don't like much, but none of the other tricks I tried helped.
2. Escape < and & to avoid HTML injection. This makes "<DELTARUNE.>" show up properly. (I had to make some additional changes to prevent double-escaping but I think I got it right.)
3. Use DOMContentLoaded instead of duplicating script.js at the start and the end of the page.
4. Add a pure-CSS fallback for highlighting the selected line (:target). It now works when JS is disabled. But the innerHTML trick from 1 breaks it so we also need the old version.

[KockaAdmiralac]

Thanks!