Do not pluralize common uncountable mass nouns (#239)

[uttam12331]

Closes #239.

Problem

plural_noun() pluralizes uncountable mass nouns that have no ordinary plural form:

>>> import inflect
>>> p = inflect.engine()
>>> p.plural_noun("equipment")
'equipments'   # should be 'equipment'

Change

inflect already keeps a small set of uncountable nouns unchanged via the "UNCOUNTABLE NOUNS" entries in pl_sb_uninflected_endings (e.g. furniture, information). Several common mass nouns were simply missing. This adds a conservative set:

equipment, luggage, baggage, jewelry, machinery, cutlery, livestock

I deliberately kept the list to words that are unambiguously uncountable (no countable sense) and are not a suffix of a common countable word, so the existing suffix matching doesn't accidentally swallow ordinary plurals. As @ecederstrand noted in the issue, words like "paper" and "sugar" have both count and mass senses, so those are intentionally left out.

Tests

Added tests/test_uncountable_nouns.py:

• test_mass_nouns_are_uninflected — each added mass noun is returned unchanged (fails on main, e.g. equipment → equipments).
• test_countable_nouns_still_pluralize — an over-match guard verifying representative countable nouns (document, element, package, stock, block, cutter, …) still pluralize normally.

Full suite passes (214 passed, 16 pre-existing xfailed). ruff format --check is clean on the changed files, and the change introduces no new ruff check findings.