Support transforming the dirname used by uniquify

Support transforming the dirname used by uniquify

By transforming the buffer's directory name, we can add
additional information to use during uniquifying.  A basic
one: uniquifying buffer names based on the project name.
* lisp/progmodes/project.el (project-uniquify-dirname-transform): Add.
* lisp/uniquify.el (uniquify-dirname-transform-default)
(uniquify-dirname-transform): Add.  (Bug#62621)
(uniquify-rationalize-file-buffer-names, uniquify-buffer-file-name):
Use 'uniquify-dirname-transform'.
* test/lisp/uniquify-tests.el (uniquify-home)
(uniquify-project-transform): Add tests.