brew-info: Add pinned status to info.

Closes #18851.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
brew-pin: Fix bug in `FormulaPin#pinned?`

Since `pinned?` was using `File#exists?`, broken symlinks left by force-upgraded
formulae made `pinned?` return false while `pin` returned with an error because
it could not overwrite the broken link. This is fixed by using `File#symlink?`
instead, which returns true for broken symlinks and false only when they do not
exist.

Closes #18858.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>