Add Sinhala (si_LK) locale

[RavinduPabasara]

Adds a Sinhala (si_LK) locale — the language of ~17 million speakers in Sri Lanka, previously unsupported by humanize.

Changes proposed in this pull request:

• Add src/humanize/locale/si_LK/LC_MESSAGES/humanize.po translating all time, number-scale and file-size phrases into formal written Sinhala, verified by a native speaker.
• Add Sinhala to the localized-languages list in README.md (alphabetical position).
• Add Sinhala coverage to tests/test_i18n.py (naturaltime, precisedelta, intword, naturalsize).

Notes for reviewers:

• Plural rule follows CLDR for Sinhala: nplurals=2; plural=(n > 1);.
• A grammatical subtlety worth knowing: Sinhala marks "ago"/"from now" with a case suffix on the duration (පැයකට පෙර = "an hour ago", පැය 2කට පෙර = "2 hours ago"). Because gettext does plain substitution and cannot perform සන්ධි (morphophonemic joining), the single-unit forms (an hour, a minute, …) are rendered as bare nouns (පැය, විනාඩිය) so they compose correctly through %s ago / %s from now. This makes every naturaltime output grammatical; the only visible effect is that a direct naturaldelta(timedelta(hours=1)) returns the bare noun පැය rather than an article form. Given naturaltime is the dominant consumer of these strings, this was the correct trade-off.

Full local run: 789 passed (including the new Sinhala i18n assertions), ruff/black clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (c3a124c) to head (24b7038).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   99.56%   99.56%           
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  Files          12       12           
  Lines         913      916    +3     
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+ Hits          909      912    +3     
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.59% <100.00%> (+<0.01%)	⬆️
ubuntu-latest	97.59% <100.00%> (+<0.01%)	⬆️
windows-latest	95.41% <0.00%> (-0.32%)	⬇️

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[hugovk]

Thank you!

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

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_{Comparing RavinduPabasara:feature/si_LK-locale (24b7038) with main (c3a124c)}