[Markdown] Add GitHub Alerts syntax

[deathaxe]

Resolves #3891

This PR proposes to add support for syntax highlighting of GitHub alerts as proposed by #3891.

This PR uses dedicated scopes markup.heading.alert and markup.quote.alert to avoid unwanted negative impacts on highlighting in conjunction with legacy color schemes. Dedicated highlighting requires to opt-in via new color scheme rules. Otherwise those blocks keep highlighting as normal block quotes except [!...] marker being highlighted as normal heading.

[jwortmann]

markup.quote.alert looks fine to me, but if you want to use markup.heading.alert for the alert type, then this scope should at least be excluded from the symbol list.

[deathaxe]

Extended the scope of exclusion to all kinds of headings in block quotes as they are unlikely to represent normal documentation structure.

[jrappen]

Reading through this, maybe change some scope suffixes here from .markdown to .markdown.github? Haven't checked if it's done elsewhere.

[deathaxe]

Maybe .markdown-gfm, which all the other Github Favoured Markdown constructs use. But actually I see barely use for that and would even rather drop all of those distinctions, especially as most (not all) of them have been adopted by CommonMark as well.

IIRC, only remaining GFM specific expressions are tables and now github alerts.

Any other admonation syntax could use exactly the same scopes. Not sure if anyone would want to highlight them different.

[jwortmann]

Just a small discrepancy that I noticed:

If there is an empty line within an alert block, the alert scope is not applied anymore. But on GitHub, an empty line doesn't close the alert block:

Not sure if this was intentionally made different in the syntax to keep the implementation simple, or if it was just missed.