Allow `:diminish` with no arguments

Allow `:diminish` with no arguments

When given no arguments, have :diminish assume it should diminish a
mode named after the current package (the package’s name, with “-mode”
appended, if it’s not already) to an empty string.

When given only a string to diminish an implicit package name to, do
not append “-mode” to the package name if it already ends with
it.  (This is a backwards-incompatible change if someone was
diminishing a package named “foo-mode” implementing `foo-mode-mode`.)

Add test cases for `use-package-normalize-diminish`.

This addresses some of the redundancy mentioned in issue https://github.com/jwiegley/use-package/issues/288.