Add category metadata to completing-read

[juergenhoetzel]

I added the metadata only to the default Emacs completing-read. as I am not familiar with the 3rd party systems helm and ivy.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the readme (if adding/changing user-visible functionality)

I'm not sure about checking the last 2 checkboxes. The changes are only visible if using third-party packages like
marginalia and embark which use the metadata.

projectile-switch-project without metadata:

with metadata:

[bbatsov]

Cool improvement! Thanks!