Ensure keyUp/keyDown value it one grapheme cluster
Return InvalidArgumentError if the action value for key actions is
not exactly one grapheme cluster long.
Supporting full grapheme clusters rather than just single codepoints
is marked as TODO in the spec, so add tentative tests for that.
Differential Revision: https://phabricator.services.mozilla.com/D217950
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1910352
gecko-commit: b39c4bd4277c4e2c1b82762f4d1fb8a5bbc34ea3
gecko-reviewers: webdriver-reviewers, jdescottes