Refactor: replace dialect_of! table partition check with Dialect trait method

[jerguslejko]

Summary

Continues the work from #2171 and #2175 by replacing one more dialect_of! macro usage with a Dialect trait method, so user-defined dialects can opt into table-factor PARTITION parsing.

New trait method:

Method	Description	Dialects
supports_table_partitions	PARTITION (p0, p1) on a table factor	MySQL

Also implemented for GenericDialect (returning true), matching the dialects the previous dialect_of!(self is MySqlDialect | GenericDialect) check named. The default is false, so behaviour is unchanged for every existing dialect.

Motivation is the one described in #1430 and #1186: a custom dialect wrapping PostgreSqlDialect cannot reach this branch at all today, because dialect_of! is a TypeId check that only matches built-in dialects.

Test plan

• All existing tests pass (1620 across 23 suites), including parse_table_partition_selection
• New test in tests/sqlparser_custom_dialect.rs: a custom dialect opting in parses and round-trips SELECT * FROM employees PARTITION (p0, p1); one that does not opt in still rejects it
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo fmt --all -- --check passes