Support configurable rounding modes in sigfig.round

[Copilot]

sigfig.round lacked directional rounding while preserving a specified number of significant figures. This adds a mode keyword supporting all eight decimal rounding modes.

• API: Export rounding constants from sigfig; apply mode to significant-figure, decimal-place, and uncertainty rounding. Keep ROUND_HALF_UP as the default.
• Documentation: Add “Rounding Modes,” distinguishing toward-zero rounding (ROUND_DOWN) from toward-negative-infinity rounding (ROUND_FLOOR).
• Coverage: Add regression cases for each mode, negative values, carry boundaries, uncertainty, and decimal-context independence.

from sigfig import round, ROUND_DOWN

round(199, sigfigs=1, mode=ROUND_DOWN)  # 100
round(999, sigfigs=2, mode=ROUND_DOWN)  # 990

• Fixes Add option to round down #59

[MikeBusuttil]

move all new test cases out of here and into test_equality.csv

[MikeBusuttil]

resolved in #64