Expose dashjs.isSupported utility for checking MSE support

Created by: chromakode

Similar to https://github.com/Dash-Industry-Forum/dash.js/issues/2055, I was looking for an easy way to test for MSE support before falling back to HLS. This small change exposes the existing supportsMediaSource check into a dashjs.isSupported() function on the global module object.

Example usage:

if (dashjs.isSupported()) {
  const player = dashjs.MediaPlayer().create()
  // ...
} else {
  // fallback logic
}