[Feature] Dynamic font replacement

[oberrich]

Currently there are multiple inherent issues with replacing the font globally:

• Icon fonts aren't exhaustively excluded, leading to replacement characters being commonly displayed (mostly fixed in [Feature] Improve symbol/icon font detection #12691)
• Complex code viewers and editors break when using a non-monospaced font (Fixed for GitHub.com in fix: update dynamic theme fixes for GitHub.com #12693)

Currently this feature is implemented naively in createTextStyle.
I think it might be interesting to expand on this feature by:

• Detecting monospaced fonts
• Detecting emoji/icon fonts

This could be done possibly by either checking the name (e.g. is font family ui-monospace, Consolas, ...`), actually analyzing the font's glyphs/code points programmatically or rendering it to see if it displays correctly. Fallback: Don't replace font.

This would allow to offer a much more granular control over font replacement, for example based on the standard CSS font-family aliases ui-*, emoji, math, etc.

Example: Chromium font selection

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Why not fix inside dynamic theme fixes?

It's not sustainable to hardcode a list of monospaced fonts for every broken site and keep that up-to-date.
Sometimes it's also non-trivial to match code blocks using CSS or HTML directly, so we need to use globs/wildcards and that's not trivial. Not overriding the font in the first place for an element that should be excluded would be ideal.