This adds HTML formatting capabilities to the formatter package. HTML is
emitted as inline styles. For palette indexes, direct RGB is emitted if
we have access to a palette; otherwise, we fall back to CSS variables.
This isn't exposed to end users yet, but will enable copy as html
features. This is available in libghostty.
Fixes#9395
**AI disclosure:** I used AI (Amp) to help me write tests, but the
implementation was done manually. I reviewed everything.
c0e483c49e