Disallow '@' as a top level syntax

The grammar does not allow '@' as a bare token.  It's only allowed
in a function call.

This is based off of the cleanup-pratt-jep-4 branch so it includes 07be644 stuff.  The actual commit is https://github.com/jamesls/jmespath/commit/dfadd3c45c80c0b8e202e8e7b9ec1d2b2002c779

cc @mtdowling.