Lower case web-features IDs

[captainbrosset]

#1012 is not getting recognized by the web-features bot because the id XSLT is upper case. All web-features IDs are lowercase.

[foolip]

I edited #1012 as that was the only way to re-run the workflow. So the immediate problem is fixed.

[jgraham]

I"d be tempted to write a helper function rather than have to remember to write toLowerCase every time you are going to check if the feature exists, even if it's something mildly unpleasant like:

function addIfExists(feature, gatheredFeatures) {
  const id = feature.toLowerCase();
  if (features[id]) {
    gatheredFeatures.add(id);
  }
}

(mildly unpleasant because it's mutating its arguments, but I think anything nicer would end up wrapping gatheredFeatures in a class, which is probably overkill for this kind of script).