addons: try to be smarter about showing file extensions

We now hide add-ons' file extensions in the commands list except when
they are needed for disambiguation. There are three cases:

- when an add-on exists with only one file extension, the extension is
  not displayed.

- when an add-on has exactly two variants, one with the .[l]hs extension
  and the other with none or .exe - presumably the source and compiled
  versions - we omit the source version from the list.

- otherwise when an add-on has multiple variants differing by file
  extension, they are listed with their extensions displayed.
tools: update test addons