Refine the color decorators scope

[jennybc]

Splitting this out from #1613. The original topic there is a feature request (that github issue numbers be auto-linked in NEWS.md).

However that provoked much discussion of generally over-eager color decoration in Positron. Some of this may be inherited from VS Code, but some of it may be due to choices we've made. See #1613 for more discussion.

This issue is for the more limited, more bug-fix-y issue of suppressing inappropriate color decoration.

[jennybc]

From the other thread, here's an example of over-eager color decoration. An issue number should not be decorated with a color picker when authoring a commit message and this appears to affect Positron but not VS Code. Screenshot from other thread, but I can also reproduce the behaviour in Positron Version: 2024.02.0 (Universal) build 1567. And in VS Code Version: 1.86.1, this doesn't happen in a git commit message. So it's something we've done.

[jennybc]

If we don't fix this, maybe we can frame this as Positron's synesthesia feature, which associates certain colors with your GitHub issues 🌈

[juliasilge]

Look at this (from Bluesky) happening for an address as well 🙈