Redshift shouldn't support window clause

[owenjonesuob]

Currently, supports_window_clause() returns TRUE for Redshift connections, because the Redshift backend inherits from the Postgres backend, and Postgres does support window clauses.

However, Redshift does not currently support window clauses - at least, they are not mentioned anywhere in the AWS Redshift docs, and attempting to use a named window in a Redshift query in the same way as in a Postgres query, results in a syntax error.

Fixable by adding supports_window_clause() S3 methods for Redshift connections in backend-redshift.R.