Pull out most xterm handling from TerminalInstance into its own class that encapsulates addons, latest write/parse, _enableWebglRenderer/other renderer stuff, customkeyhandler?, etc. The goal is to make TerminalInstance much smaller and enable easy unit testing of previously core TerminalInstance behavior by not having so many service dependencies.

• Expose xterm as public ITerminalInstance to avoid creating passthrough functions like scrollToTop
• Expose xterm.raw as public but only to TerminalInstance (ie. don't expose on XtermTerminal's interface)
• TerminalInstance will be mostly a glue class (process manager <-> xterm), not intended to be unit testable but by integration tests