Provide with "15" and "15-graphemes" UnicodeProviders

Th former doesn't support grapheme clusters, except the old-fashioned
kind of simple modifiers.  However, "15-graphemes" (with full
cluster support) is the default.

addons/xterm-addon-unicode-graphemes/test/UnicodeGraphemesAddon.api.ts

@@ -39,7 +39,7 @@ describe('UnicodeGraphemesAddon', () => {
       window.term.loadAddon(window.unicode);
     `);
     // should have loaded '15-graphemes'
-    assert.deepEqual(await page.evaluate(`window.term.unicode.versions`), ['6', ourVersion]);
+    assert.deepEqual(await page.evaluate(`window.term.unicode.versions`), ['6', '15', '15-graphemes']);
     // switch should not throw
     await page.evaluate(`window.term.unicode.activeVersion = '${ourVersion}';`);
     assert.equal(await page.evaluate(`window.term.unicode.activeVersion`), ourVersion);