Support `PREFIX` directives in turtle parsers.

[damooo]

Currently turtle parsers seems doesn't take PREFIX case-insensitive sparql-like directives, which are supported by turtle syntax since 2014. It would be highly valuble for their support to handle turtle from wild.

[TallTed]

Important for implementation —

• @prefix (with @) declarations ARE case sensitive.
• PREFIX (without @) declarations ARE NOT case sensitive.

From the Turtle 1.1 spec —

The Turtle language originally permitted only the syntax including the '@' character for writing prefix and base directives. The case-insensitive 'PREFIX' and 'BASE' forms were added to align Turtle's syntax with that of SPARQL.