Add lexer for ShExC

ShExC [1] is one syntax for the ShEx (shape expressions) language [2] to
describe the structure of RDF graphs (the other two syntaxes are based on
JSON-LD and RDF and don?t need special lexers). It is syntactically similar to
SPARQL, which is why a lot of the productions of ShExCLexer are copied from
SparqlLexer, but at the same time has enough differences that I feel it?s
better to simply copy the productions rather than trying to share them between
the two lexers (compare e.?g. PN_LOCAL_ESCAPE_CHARS or IRIREF).

The example file purports to be a brief schema for Pygments lexers, which I put
together from scratch to avoid licensing issues with existing example schemas;
it should not be taken too seriously.

[1]: https://shex.io/shex-semantics/#shexc
[2]: https://shexspec.github.io/primer/

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