feat: replace git prompt with starship

[dmccaffery]

• remove installation of git prompt
• add installation of fira code nerd font (required by starship)
• add installation of starship
• set default starship config
• replace default prompt with starship
• update dark-mode theme to use fira code nerd font
• update preview image in readme
• add installation of github cli

Closes: #76

[dmccaffery]

@sjk07 : you can clone my fork and run ./install.sh from my branch if you want to test before this is merged:

[dmccaffery]

:tada: This PR is included in version 8.3.0-next.1 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: