Modify `Dialect` trait to include `supports_group_by_expr` function

[jdye64]

The current Dialect trait has no mechanism for determining if the implementing instance supports Grouping Exprs. This means that any implementation of the Dialect trait has no way to inform sqlparser-rs that it supports the Rollup clause for example.

Adding a function to Dialect like already existing functions, supports_filter_during_aggregation, for example would allow the implementation to inform the parser if it does in fact support grouping exprs or not.

The list is currently hardcoded in parser.rs to if dialect_of!(self is PostgreSqlDialect | DuckDbDialect | GenericDialect)