Add curly bracket to JS provider for use in React.

[bilherron]

This PR partially addresses https://github.com/zignd/HTML-CSS-Class-Completion/issues/287 by updating the regex that matches for JS(X)/TS(X) files.

The regex uses a non-capturing group (?: to match an optional opening curly bracket before a single or double quote to trigger auto-completion. Matches the following:

className="...
className='...
className={"...
className={'...
class="...
class='...
class={"...
class={'...