Xontrib-prompt-starship

In case of [using starship as part of another prompt](https://github.com/anki-code/xontrib-prompt-bar/blob/master/README.md#using-starship-cross-shell-prompt-for-rendering-sections) you can add starship prompt to `$PROMPT_FIELDS` without replacing the current prompt:
python
$XONTRIB_PROMPT_STARSHIP_REPLACE_PROMPT = False
xontrib load prompt_starship
print($PROMPT_FIELDS['starship_left']())

Added `XONTRIB_PROMPT_STARSHIP_BOTTOM_CONFIG`.

Added RIGHT_PROMPT support. Thanks to eugenesvk!

Added setting `STARSHIP_SESSION_KEY`.

Fixed reading return code for `xontrib load` command.

* Fixed starship encoding when it already init in outside shell
* Fixed duration