Add getDuration for parsing time strings to milliseconds

[recursivefunk]

Summary

Adds getDuration(key, defaultVal) (and the duration() alias) for parsing human-friendly time strings like "500ms", "30s", "5m", "2h", "1d", "1w" into milliseconds. This covers a very common env-var pattern (timeouts, TTLs, intervals) that today requires hand-rolled parsing on top of getNumber. Additive and non-breaking — bumps minor to 7.7.0.

Changed Files

• src/index.js — new DURATION_UNITS, DURATION_RE, and parseDuration helpers plus getDuration/duration methods. Default-fallback behavior mirrors getIP (invalid env value → fall back to parsing the default).
• src/index.d.ts — type declarations for getDuration and duration with a number | null return.
• test/test.env — fixtures covering every unit, decimals, uppercase unit, and whitespace between number and unit.
• test/test.js — seven new test blocks (18 assertions) covering every unit, decimals/case/whitespace, invalid values, default fallback, missing-key cases (string default, numeric default, no default), unparseable defaults (string, NaN, {}), and the alias.
• README.md — new example in the Type Conversion section.
• package.json — 7.6.3 → 7.7.0.

Steps to Verify

• npm run ci — all 116 tests pass and 100% line coverage still holds.
• env.getDuration('CACHE_TTL') with CACHE_TTL=5m returns 300000.
• env.getDuration('UNSET', '30s') returns 30000.
• env.getDuration('UNSET', 1000) returns 1000 (numeric default treated as ms).
• env.getDuration('UNSET') returns null.
• env.getDuration('INVALID') where INVALID=nope returns null; same call with a valid string default returns the parsed default.
• env.duration('CACHE_TTL') matches env.getDuration('CACHE_TTL').