Enforce UTF-8 when url is given to the basic URL parser

[annevk]

HTML stopped using the encoding argument under these conditions as of whatwg/html#8309 as far as I can tell. Making that clear in the specification would benefit implementers.

[annevk]

Here's a somewhat ambitious idea:

• We rename basic URL parser to internal URL parser and mandate all of its arguments. We stop exporting it.
• We create a new API URL parser that is essentially the internal URL parser except it does not have an encoding argument. This one is exported. (The existing internal API URL parser would have to be renamed or maybe this should have a slightly different name.) This would require a whatwg/html PR.
• We modify the URL parser entry point so that it is suitable for all implementations, perhaps by making the blob: URL and encoding requirements conditional upon the user agent being a browser.

A less ambitious approach would be to add some asserts.