Move the PostgreSQL skip methods from the PostgreSQL parser to the
abstract parser. This is step 1 in refactoring the GoogleSQL and
PostgreSQL parser so they can share more code. The eventual goal is to
allow the GoogleSQL parser to be able to handle SQL string without
having to remove the comments from the string first.

Generalize the various skip methods so these can be used for both
dialects. Each dialect implements a number of abstract methods to
indicate what type of statements and constructs they support. These
methods are used by the generalized skip methods to determine the start
and end of literals, identifiers, and comments.