Skip reserved-function-name check for URIQualifiedLiteral

[GuntherRademacher]

The XQuery spec, since 1.0, contains a constraint named reserved-function-names with a formulation like this or similar:

Unprefixed function names spelled the same way as language keywords could make the language impossible to parse. For instance, element(foo) could be taken either as a FunctionCall or as an ElementTest. Therefore, an unprefixed function name must not be any of the names in A.4 Reserved Function Names.

The current implementation in BaseX focuses on "unprefixed" and thus tests the absence of a prefix. This may result in a URIQualifiedName being rejected, if its local name is in the list of reserved names.

I do not consider this as correct. The motivation behind the constraint is the ambiguity that arises when a plain name is used. For a URIQualifiedName, this ambiguity does not exist - the Q{uri}local-name form unambiguously stands for a QName.

I thus propose to make the implementation focus on the spec's wording "function names spelled the same way as language keywords", and treating URIQualifiedNames as outside the constraint's scope.

This is implemented in this PR.