[LJimmy007]

Fixes #550.

With reindent=True, a date scalar with an explicit alias splits the SELECT
projection list. For example,
SELECT 'value 1' AS VALUE_1, CURRENT_DATE AS CREATED_AT, 'testing' AS VALUE_2, 'testing2' AS VALUE_3 FROM TABLE_1
indents the last two columns by 23 spaces instead of 7.

Allow CURRENT_DATE, CURRENT_TIME, and CURRENT_TIMESTAMP in the existing
explicit-AS grouping rule, alongside NULL. This keeps each scalar with its
alias, restores column alignment, and makes get_alias() return CREATED_AT.
Other keyword handling and implicit aliases are unchanged.

Tests cover all three scalars in upper/lower/mixed case, ordinary-column and
NULL controls, exact parsing round trips including whitespace, projection
boundaries, alias discovery, and repeated formatting. The reported trailing
comma is covered separately for preservation; the main regression uses valid SQL.

Validation on Windows / Python 3.11.17:

• The same 25 regression/control cases: 19 failed, 6 passed on the original
  implementation; all 25 pass with this change.
• uv run --group dev --python 3.11 pytest tests/test_grouping.py tests/test_format.py:
  190 passed, 2 xfailed.
• uv run --group dev --python 3.11 pytest tests/:
  531 passed, 2 xfailed, 1 xpassed. The xfail/xpass outcomes match unchanged
  upstream (506 passed, 2 xfailed, 1 xpassed).
• uv run --group dev ruff check sqlparse/ and git diff --check pass.

Developed with AI assistance and independently reviewed by another agent, which
also reran the checks above. Other Python versions/platforms were not tested
locally for the complete suite.

• ran the tests (pytest)
• all style issues addressed (ruff)
• changes covered by tests
• changelog updated