Redshift dialect: support nested block comments #2959

Redshift nests /* */ comments, as Postgres does, but RedshiftSqlDialect doesn't set supports_nested_comments, so sqlparser ends the comment at the first */.

select 1 /* /* */ , ' */ , pg_backend_pid() -- '

• sqlparser 0.64.0: MultiLineComment(" /* "), then a string; parses as SELECT 1, ' */ , pg_backend_pid() -- '.
• Redshift: two columns, 1 and 1073905994, so the comment ran to the second */.

The overlapping form select 1 /*/*/ , ' */ */ , pg_backend_pid() -- ' behaves the same way on both sides.

Postgres documents nesting in "Comments" (https://www.postgresql.org/docs/8.0/sql-syntax.html#SQL-SYNTAX-COMMENTS), and Redshift is based on PostgreSQL 8.0.2.