Shared: Provide custom isURI that rejects unknown schemes [isURI]

We also export the set of known `schemes`.

The new function replaces the function of the same name
from `Network.URI`, as the latter did not check whether a scheme is
well-known.  E.g. MediaWiki wikis frequently feature pages with names
like `User:John`. These links were interpreted as URIs, thus turning
internal links into global links. This is prevented by also checking
whether the scheme of a URI is frequently used (i.e. is IANA registered
or an otherwise well-known scheme).

Fixes: #2713

Update set of well-known URIs from IANA list
All official IANA schemes (as of 2017-05-22) are included in the set of
known schemes.  The four non-official schemes doi, isbn, javascript, and
pmid are kept.

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