Add customElements.getName()

[mash-graz]

Fixes: #3640

[domenic]

This is missing tests. Fortunately, after 63265a9, you can probably follow the procedure outlined in https://github.com/jsdom/jsdom/blob/main/Contributing.md#web-platform-feature-tests to enable a preexisting test.

[mash-graz]

Although I could find already existing tests for getName in unmerged WPT branches, I can't simply enable them, because they are not available in the main branch of the WPT upstream repo or here in the test directory.

see:

web-platform-tests/wpt@a644106#diff-308a795609efdc52e2aee7ccd0ee171e4e1bd72a034183c807f81ee6dbdb0d61

web-platform-tests/wpt#39640

[asamuzaK]

It looks like file moved.
See Move CustomElementRegistry.getName() tests to a tentative test file · web-platform-tests/wpt@99ee1f5