Group date-part aliases in SELECT projections

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Fixes #781.

SELECT EXTRACT(YEAR FROM foo) year, EXTRACT(MONTH FROM foo) month, value FROM orders now produces one projection list with year and month aliases. Previously the aliases split the projection into separate groups and get_alias() did not return them.

Recognize the six date-part keywords only after a bare function at a SELECT-item boundary, with a following projection or clause boundary. Keep interval constructors and their units intact, including INTERVAL (ABS(-1)) DAY; quoted function names remain distinguishable. The tests also cover scalar-literal predecessors, explicit aliases, nested SELECTs, window functions and JOIN/UNION clauses.

This is a bounded grouping fix. Unknown clause keywords stop recognition; comment-separated aliases and broader keyword-alias grammar remain outside its scope.

On Python 3.13.0, the full suite reports 555 passed, 2 expected failures and 1 unexpected pass; the expected-failure statuses match upstream. Against the original source, 40 alias regression cases fail, and an additional formatter regression reproduces the excessive indentation in #781. ruff check sqlparse/ passes. The repository's grouping benchmarks and a function-alias projection benchmark report linear scaling. The other Python/OS combinations are left to hosted CI.

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