Add features for <input type="file" multiple> and<input type="file" webkitdirectory>

[ddbeck]

This creates features that handle uploading files and directories with form inputs. They map to:

• https://caniuse.com/input-file-directory and inspired by this web.dev blog post
• https://caniuse.com/input-file-multiple and the scenario described by @captainbrosset in File and directory entries API #2993 (comment)

This approach treats these attributes as a distinct input type, like we do for various <input type="…"> scenarios, though it's admittedly weird for it to be via an attribute.

While I was here, I sifted through draft/file-and-directory-entries.yml, disposing of the keys which were already assigned. This left me with almost nothing. The remaining exceptions largely have poor data quality in BCD (see mdn/browser-compat-data#19304) and won't work as a "feature" until that's resolved.

Closes #3438, closes #2993, and closes #2633.

[ddbeck]

OK, let's merge this now. If there's more to do around webkitEntries, we can do it in a follow up. Thank you!