refactor: call match.group once per field in __format__

[underwoo]

Fixes #6

Each named regex group was fetched twice — once for truthiness, once for the value. Fixed with or operator:

# Before
fill = match.group("fill") if match.group("fill") else fill

# After
fill = match.group("fill") or fill

(width still uses the conditional form since int(None) raises.)

Verification:

• pytest: 47/47 passed
• mypy: clean
• pylint: clean

Closes #6