Add support for postgresql tablesample command

Postgresql supports an optional `[ TABLESAMPLE sampling_method ( argument [, ...] ) [ REPEATABLE ( seed ) ] ]` during a select statement to retrieve a subset of rows based on the arguments provided.

Start at the SELECT-statement parser and inspect how PostgreSQL-specific clauses are handled. Add support for the TABLESAMPLE syntax described in the issue, including its sampling method, arguments, and optional REPEATABLE seed. Run the existing parser test suite and verify that valid PostgreSQL statements parse successfully.

Written by the indexing model from the issue text.