Adds 256 color and truecolor support to vterm. Fixes #457

[danschwarz]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment [Python 3.x only; can't run 2.7 locally]
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Adds high color support to vterm.py - 88 color, 256 color, truecolor modes.
Monochrome and 16 color mode work as before.

What's missing/needed:
Testing against Python 2.7 - my Ubuntu 22.04 dev box complains when running 2.7, which causes tox to fail.
Add tox tests if it's possible to verify correct color output using tox tests?

Here is palette_test.py running in terminal.py:

[penguinolog]

please fix attr 39 parsing and LGTM

[penguinolog]

I will run tests against dependent projects in few hours and merge on success

[penguinolog]

khal tests pass
zulip-terminal tests pass