Check LSL syntax file for compatibility

The LSL syntax file should have a mechanism for checking that it is compatible with the viewer.

I suggest another element in the top-level map whose key is "llsd-lsl-syntax-version" and value is an integer.

This can be used to detect whether or not the format of the syntax file has changed (not its content... the server-generated hash id does that).

The viewer code should be modified to ignore any returned syntax file that does not have this attribute, which will server to ignore the version that does not have the STORM-2000 fix.