Add option to set root user prompt symbol

[matchai]

Feature Request

Add configuration options to set the prompt character for the root user:

• SPACESHIP_CHAR_SYMBOL_ROOT to change the root prompt

The prompt defaults to $SPACESHIP_CHAR_SYMBOL, so this change should not alter the current behavior and not affect any users that don't change those vars.

Describe the solution you'd like
A port of the existing implementation in the following Spaceship PR: spaceship-prompt/spaceship-prompt#461

[kulabun]

There is not such thing like secondary prompt in fish and doesn't seems to appear in nearest future:
fish-shell/fish-shell#3382
So this feature will only include:
SPACESHIP_CHAR_SYMBOL
SPACESHIP_CHAR_SYMBOL_ROOT

[matchai]

Good catch @kulabun! 👀
I'll update the issue to suit what would be possible in fish.