Add ".pgsql" as a recognised PL/pgSQL extension

[Alhadis]

This PR adds .pgsql to the list of supported PL/pgSQL extensions.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension
      ~36,235 results, with no conflicting cases. This may impact 69 repositories with README.pgsql files, however.
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source
    • Sample license: CDDL (embedded in the file)

[pchaigno]

LGTM!