Fixes to `brew-pin`.

Glad to see #18515 merged. This pull request contains two minor fixes/additions to the `pin` mechanic:
* Add pinned status to `brew info`. Closes #18851.
* Fix a 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.