Correct past and future directions for datetime durations

[saitakarcesme]

Fixes #246.

With add_direction=True, a datetime one hour in the past currently produces in 1 hour, while a future datetime produces 1 hour ago. Subtract the current time from the supplied datetime so its sign follows Babel's duration convention. Direct timedelta values retain their existing meaning; output without direction stays the same.

Tests cover past/future datetime inputs, positive/negative timedelta controls, the Jinja timedeltaformat filter, and output without direction. The clock is fixed without adding a test dependency. The docstring and changelog describe the corrected behavior.

This addresses direction separately from the timezone-aware datetime support proposed in #233; that PR currently retains the old subtraction order. No timezone-awareness changes are included here.

Validation (macOS, Python 3.13.5, Flask 3.1.3, Babel 2.18.0, pytest 7.4.4):

• Before: both new datetime direction cases fail; timedelta controls pass.
• After: all 8 date-formatting tests pass.
• Full suite: 34 pass, 3 fail. An untouched git archive HEAD baseline has 30 pass, the same 3 fail: test_list_translations, test_list_translations_default_locale_exists, and test_multiple_directories. All three assume directory iteration order; the existing Fix list-translations() ordering in tests #242 addresses ordering in these tests.
• Black 24.10.0 and whitespace checks pass.
• Sphinx 8.2.3 documentation build with -W passes.

Prepared and tested with OpenAI Codex.