Use vscode-html-languageservice options to allow custom elements

[bioub]

I'm thinking of the easiest way to supports collections of custom elements.

vscode-html-languageservice offers the ability to register custom tags and attributes here :
https://github.com/Microsoft/vscode-html-languageservice/blob/62efc4b0768640e0c1f5f67974205e0c4abbfaf7/src/htmlLanguageService.ts#L46

What would you think about adding and option to htmlDefaults like this ?

monaco.languages.html.htmlDefaults.setOptions({
  customTags: {
    'staytuned-player': {
      label: 'Staytuned.io podcast player',
      attributes: ['src', 'fixed'],
    },
  },
});

Tell me if you'd like me to do the PR.

[octref]

Keep in mind the html-languageservice API is not finalized yet...

[aeschli]

duplicate of #2284

I added new API to allow configuring the HTML and CSS language supports with custom data.
See #2284 (comment) for details.