feat: support current-team filtering in load_teams

[ArtMoreno]

load_teams() currently returns historical and alternate team abbreviations and does not accept the current argument available in nflreadr. Add current: bool = True, matching the R implementation's default and its use of the existing team_abbr_mapping values.

Closes #48.

With the live team dataset checked September 9, 2026:

import nflreadpy as nfl

nfl.load_teams()               # 32 rows: standardized current teams
nfl.load_teams(current=True)   # same result
nfl.load_teams(current=False)  # 36 rows: original unfiltered behavior

The current dataset's excluded abbreviations are LAR, OAK, SD, and STL; the standardized Rams abbreviation is LA. Filtering derives from the bundled mapping, not a hardcoded list or team count. The downloaded/cached frame is preserved for callers requesting all entries.

Changes

• src/nflreadpy/load_teams.py: optional filter using the existing mapping; document the argument and default.
• tests/test_integration.py: regression coverage for default, explicit true, and false; check row order, metadata, and preservation of the unfiltered frame.
• CHANGELOG.md: explain the new default and the current=False migration path.

Validation

Native Windows, Python 3.11.15; locked development dependencies.

• New regression: all three cases fail against original main and pass with the change.
• python -m pytest tests -v: 51 passed, including live-data integration tests.
• Separate live-data check: 32 default rows, 36 unfiltered rows; repeated unfiltered calls retain the complete dataset.
• Ruff lint and formatting on changed Python files: pass.
• MkDocs build: pass; the existing utils_date.py missing-annotation warning remains.
• git diff --check: pass.
• mypy src: 5 errors in unchanged datasets.py and utils_date.py. Repeating mypy with the original load_teams.py produces the same 5 errors; no new errors from this patch.

Base: c54316e16fb6cbbfbc52ed2a9c07fca070a0c636.

No overlapping PR appeared in the all-state current load_teams search. This deliberately changes the no-argument default to match nflreadr; callers needing historical/alternate rows should pass current=False.

Prepared with Codex assistance; all reported tests were run locally.