Implement indent guide colorization options.

[mwerschy]

Resolves #158242. Apparently, this isn't possible to do properly in extensions, c.f. oderwat/vscode-indent-rainbow#3.

Broken colorization with extension:

With native implementation:

This likely breaks themes trying to use the single option name. What is the intended migration path for settings?

[mwerschy]

@microsoft-github-policy-service agree

[hediet]

@aeschli what's the recommended way to rename color variables?

[aeschli]

If the existing color is not yet used very much, breaking is ok.
If it is part of several themes, I suggest that the new color(s) use the old color as default value. The old color can be marked deprecated.

[mwerschy]

Well, I have no idea how widely used it is. They're marked as deprecated and used as defaults now.

[Igorgro]

Probably default guides colors should match brackets colors (yellow, purple, blue)