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.
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