Allow unsetting extending ansi colors

[silamon]

Following up PR to address review comments in #3905.

[Tyriar]

@jerch in case I missed anything 😛

[jerch]

LGTM 😺

There is just one thing idk for sure atm - when I was doing the OSC 4 implementation with hot color changes, there was an awkward color object reference thingy going on (not sufficiently re-grabbing colors from changes on individual slots). I am pretty sure I fixed it, still the issue kept me scratching my head back then.

So to be on the safe side - it might be good idea to explicitly check, whether a setTheme call works as before, and also that extended colors shift properly. The old issue only occured with canvas and webgl renderer.

[Tyriar]

Tested running these on each renderer and it worked as expected:

term.options.theme = { extendedAnsi: ['#ffffff']};

term.options.theme = { };