SGR color codes not supported

I'm not sure if this needs to be mushed into `ColorEscapeCode` or if it can be broken out into an `SGREscapeCode` class.

I don't think you can tell them apart until you're too far in the weeds of processing them, so it needs to go in `ColorEscapeCode`

Start by locating the ColorEscapeCode class and the processing path where escape codes are handled. Determine how SGR color codes fit into that path, then confirm that the supported SGR sequences are converted correctly to HTML.

Written by the indexing model from the issue text.