(#10146) - is not legal in variable names.

When a bunch of refactoring went on to clean up the grammer and lexer, the -
character became a legal character in variable names. This was not entirely
desired - and contravenes our existing rules for naming.

This adds testing for legal variable names, then applies that across the lexer
to ensure that legal names pass, and illegal names fail.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>