Accept URLs without a scheme in the URL link target of the BlockNote link modal

The URL link target of the BlockNote link modal only accepts full URLs: the browser rejects the URL field until the URL has a scheme (e.g. https://). So typing or pasting www.xwiki.org or xwiki.org/download doesn't work, and neither does a relative URL like /xwiki/bin/view/Main/.

Proposed behavior:

• A URL that starts with a host name (e.g. www.xwiki.org, xwiki.org/download, localhost:8080/xwiki) is completed with the https:// scheme. The URL field shows the completed URL once you leave it, so you see what will be inserted.
• A relative URL (starting with /, ./, ../, ? or #) is kept as typed.
• A URL without a scheme is saved as a typed URL resource reference (e.g. url:/xwiki/bin/view/Main/), so it's still a URL link when you edit it again. Untyped, it would be parsed back as a page reference.