lib/helpers: new function to set BASH_IT_HOMEBREW_PREFIX

[gaelicWizard]

Description

New function _bash_it_homebrew_check() sets global variable $BASH_IT_HOMEBREW_PREFIX using brew --prefix if brew exists as a valid command. If brew isn't installed, then return failure.

Plugins can test for brew by calling this function and, if it succeeds, they can rely on $BASH_IT_HOMEBREW_PREFIX being defined properly.

Motivation and Context

I already started looking at this before seeing that my PR fixes #1576. This is part of a patchset to reduce external binary invocations during shell startup. This patch doesn't actually call this function anywhere, on purpose. I don't want to load ruby if none of the user's plugins need it.

How Has This Been Tested?

Tested locally.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

Very nice and clean PR!
I will let others chime in if they want 😄

[NoahGorny]

we should maybe add some tests here to verify the logic wont be broken in the future. testing _bash_it_homebrew_check sounds ideal- by adding mock brew and removing it and seeing whats happening (even one calling bash-it reload)

[NoahGorny]

well done @gaelicWizard 😄