Improve mw.text.nowiki

The following are now correctly escaped:
* Space at the start of a line
* Start-of-line characters after \r
* Magic links such as "RFC 123" with non-space whitespace
* URIs that don't use "://", such as "urn:foo"
* Double-underscore magic words

Bug: 53658
Change-Id: I824417e2937dd27cd1e69bd4e74ab7d21a978c75
Merge "Improve mw.text.nowiki"