[FEATURE REQUEST] Install script improvements (warn if already installed, --check flag)

[connorgmeehan]

What is the feature, what would you like to be able to do?

Currently install script will backup current doom-nvim config if user tries to re-run install script. The script should check if doom-nvim is already installed and then ask the user if they want to continue.

• If the user doesn't continue the install script should still check the dependencies
• If run with the --check flag the script should just verify the user has all the dependencies they need.

What is the use case, why do you want this feature?

Improves usability of doom-nvim, easier for users to bugfix own problems.

Possible Implementation (if applicable)

Check if git repo at ~/.config/nvim's origin is doom-nvim origin.

Additional context