Add support for multi-param OSC sequences

@vapier wrote:

i don't think this correctly handles chaining to following OSC sequences.

That's correct. It doesn't handle chaining at all. It also doesn't handle color names.

Since this was my first PR, I was trying not to rewrite, but to just add a little on top of what was there and learn how things work -- I even reused the color parser that was written for OSC 4, though it won't support color names, meaning you can only set RGB values, and not just reuse colors from your base 16 for foreground and background.

The structure of the OSC handler in the State Machine seems designed on the assumption that they are single-valued (meaning one value per escape sequence), which makes chaining basically impossible.

Basically, still to do:

• Rewrite the ActionOscDispatch to support chains of values
• Make OSC 4 support setting the whole color palette at once (This was done in #7578)
• Make OSC 10, 11, 12 support setting all of them at once
• Add support for lookups ("?") to output the current value #3718
• Add support for resetting colors (OSC 104..119) #3719
• Rewrite the color parser to include x11 color names more formats #3715
• Support setting colors above index 16 #3717 (maybe #313?) Those things are definitely needed, but didn't seem necessary for a first pass.

Somewhere along the way someone will need to decide if we're going to do special colors for bold/italic/underline/blink/reverse ... and which window properties should be settable by OSC 3 😉 @oising?

Originally posted by @Jaykul in https://github.com/microsoft/terminal/pull/891#issuecomment-494667974