refactor prompt-creation-code, make modules more configurable

[betaboon]

• Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines

• [x] Docs have been added / updated in README.md (for features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

  • make the code taking care of composition of the prompt more clear
  • make it more configurable

• What is the new behavior (if this is a feature change)? New configuration options:

  • ALIEN_SHOW_EXIT_CODE - show the numeric exit-code
  • ALIEN_SHOW_DATE_TIME - completely disable the date/time-section
  • ALIEN_SHOW_BATTERY - completely disable the battery-section
  • ALIEN_SHOW_USER_HOST - completely disable the user/host-section
  • ALIEN_SHOW_HOST - show the hostname
  • ALIEN_LONG_PATH - show the full path
  • ALIEN_PROMPT_SYM - change the symbol of the prompt
  • ALIEN_SECTION_SEP_SYM - change the symbol of the separator between sections

• Does this PR introduce a breaking change? (What changes might users need to make in their configuration due to this PR?)

  • every new option is enabled by default

• Other information:

[betaboon]

I am considering to change the variable-names again and introduce some more do better configure colors of sections.

like this:

• ALIEN_SECTION_DATE_TIME_ENABLE
• ALIEN_SECTION_DATE_TIME_FORMAT
• ALIEN_SECTION_DATE_TIME_FG or ALIEN_SECTION_DATE_TIME_FOREGROUND
• ALIEN_SECTION_DATE_TIME_BG or ALIEN_SECTION_DATE_TIME_BACKGROUND

and so on and so forth.

[betaboon]

I just added the aforementioned changes on a branch in my fork: https://github.com/betaboon/alien/tree/fork

[betaboon]

btw i took inspiration for the user+host section at @Diagonactic fork: https://github.com/Diagonactic/alien/blob/master/alien.zsh#L29