Don't reject a document that starts with a "<?xml…" processing instruction

[spokodev]

Problem

consumeXmlDeclaration matches the 5-character literal <?xml and then requires
whitespace, so a document that begins with a processing instruction whose
target merely starts with xml is forced down the XML-declaration path and
rejected:

parseXml('<?xml-stylesheet type="text/xsl" href="a.xsl"?><a/>');
// throws: Invalid XML declaration (line 1, column 6)
parseXml('<?xml-model href="a.rng"?><a/>');
// throws: Invalid XML declaration (line 1, column 6)

These are well-formed documents, and both <?xml-stylesheet?> and <?xml-model?>
are extremely common in the wild (RSS/Atom, SVG, any XML+XSLT/CSS document). The
same PI parses fine when it isn't first:

parseXml('<?xml version="1.0"?><?xml-stylesheet href="a.xsl"?><a/>'); // OK

Per XML 1.0:

• An XML declaration is '<?xml' VersionInfo … where VersionInfo ::= S 'version' …
  — the <?xml must be followed by whitespace.
• PITarget ::= Name - (('X'|'x')('M'|'m')('L'|'l')) subtracts only the exact
  3-letter name xml; xml-stylesheet / xml-model are distinct, valid Names.
• prolog ::= XMLDecl? Misc* and Misc ::= Comment | PI | S (§2.8) allow a
  processing instruction as the first item when no declaration is present.

The names-beginning-with-xml-are-reserved note (§2.3) is a standardization
reservation, not a well-formedness constraint — processors must accept such names
(the same reason xmlns is legal).

Fix

After consuming <?xml, if the next character is a name character this is a PI
whose target begins with xml: rewind and let it be parsed as a processing
instruction (the caller's Misc* loop handles it). <?xml followed by
whitespace (a real declaration) or by a non-name character (e.g. <?xml?>, whose
bare xml target is reserved and has no valid interpretation) is unchanged.

Test

Added a case asserting <?xml-stylesheet?> at document start parses as a PI with
the root element following. Repointed the existing "invalid XML declaration" test
to <?xml?> (still correctly rejected). Full suite green (1380 passing) at 100%
coverage.

[rgrove]

Thanks!