Move postgres specfiic code into postgres dialect

The recent PR for postgres support has led to postgres specific parsing and writing code creeping into the base sqlparser module. We need to move the postgres specific code into the postgres dialect module.

Start by locating the base sqlparser module and the postgres dialect module, then trace the PostgreSQL-specific parsing and writing code mentioned in the issue. Done means PostgreSQL-specific behavior resides in the postgres dialect while the base module remains dialect-independent; run the existing parser tests if available.

Written by the indexing model from the issue text.