feat: Implement dynamic left and right prompt prefix options

[reobin]

Closes #148

4 "new" options were implemented:

• TYPEWRITTEN_LEFT_PROMPT_PREFIX (existed before)
• TYPEWRITTEN_LEFT_PROMPT_PREFIX_FUNCTION
• TYPEWRITTEN_RIGHT_PROMPT_PREFIX
• TYPEWRITTEN_RIGHT_PROMPT_PREFIX_FUNCTION

The _FUNCTION variants are used to enable the use of live evaluated functions as prefixes.

Example of use in .zshrc to display kube context:

display_kube_context() {
  tw_kube_context="$(kubectl config current-context 2> /dev/null)"

  if [[ $tw_kube_context != "" ]]; then
    echo "($(basename $tw_kube_context))"
  fi
}

export TYPEWRITTEN_LEFT_PROMPT_PREFIX_FUNCTION=display_kube_context

[reobin]

Hello @KEDLogic ! If you have the chance to review this PR, it would be greatly appreciated. Especially the example snippet for kube context.

[KEDLogic]

Looks good!
It just needs a small edit to the kubecontext snippet as noted.

[reobin]

@all-contributors please add @KEDLogic for documentation and ideas

[allcontributors]

@reobin

I've put up a pull request to add @KEDLogic! 🎉