Updates theme names to be compatible with Godot 3.5 (no spaces),
consistent with other Godot theme item names (snake_case), and
match the color names listed on the
[ANSI escape code wikipedia page](https://en.wikipedia.org/wiki/ANSI_escape_code#3-bit_and_4-bit).
Deprecates the old names and warns users to change them.
Commit GUT directly to the git repo and remove gd-plug as it is no
longer required to install GUT.
Modify GUT to be used by command-line only.
For example:
```
cp test/.gutconfig.ci.json .gutconfig.json
godot --no-window -s addons/gut/gut_cmdln.gd
```
