Add getDate for parsing date strings to Date objects

[recursivefunk]

Summary

Adds getDate(key, defaultVal) (and the date() alias) for parsing ISO 8601 date strings into native Date objects. This covers the common env-var pattern of release/deploy times, expiration dates, and schedule anchors. Additive and non-breaking — bumps minor to 7.8.0.

Changed Files

• src/index.js — new parseDate helper plus getDate/date methods. Uses Date constructor semantics, validates via Number.isFinite(d.getTime()). Default-fallback behavior mirrors getIP and getDuration.
• src/index.d.ts — type declarations for getDate and date with a Date | null return.
• test/test.env — fixtures for a valid ISO datetime, a valid date-only string, and an invalid value.
• test/test.js — seven new test blocks (14 assertions) covering ISO datetime parsing, date-only parsing, invalid env values, default fallback, missing-key cases (string default, Date instance default, no default), unparseable defaults (garbage string, invalid Date, number, object), and the alias.
• README.md — new example in the Type Conversion section.
• package.json — 7.7.0 → 7.8.0.

Design Notes

• ISO 8601 is recommended. The parser calls new Date(input), so anything the Date constructor accepts will work. Strings without a timezone are interpreted as local time per ECMA-262 — consumers should prefer ISO strings with Z or +HH:MM for portability.
• Numbers are intentionally rejected. Storing a Unix timestamp as an env var and reading it through getDate is ambiguous (seconds vs. milliseconds) and mixes well-typed and stringly-typed behavior. Consumers who need that can env.getNumber('TS') and pass to new Date() themselves, or supply a Date instance as the default.

Steps to Verify

• npm run ci — all 130 tests pass and 100% line coverage still holds.
• env.getDate('RELEASE_AT') with RELEASE_AT=2024-01-15T10:30:00Z returns a valid Date at that instant.
• env.getDate('UNSET', '2024-01-01') returns a Date at 2024-01-01 UTC.
• env.getDate('UNSET', new Date()) returns that Date instance unchanged.
• env.getDate('UNSET') returns null.
• env.getDate('INVALID') where INVALID=nope returns null; same call with a valid default returns the parsed default.
• env.date('RELEASE_AT') matches env.getDate('RELEASE_AT').