Add toHuman: describe a cron expression in English

[merencia]

Adds the reverse direction: toHuman(cron) turns a 5- or 6-field node-cron expression back into plain English. It's the readback for a toCron call ("you wrote X, here's the cron, which reads as Y").

What it does

import { toHuman } from 'cron-translate';

toHuman('0 0 9 * * *');     // "at 9am"
toHuman('0 0 18 * * 1-5');  // "at 6pm on monday to friday"
toHuman('0 */5 9 * * *');   // "every 5 minutes at 9am"
toHuman('0 0 0 * * 5L');    // "last friday of the month"

Covers frequency, clock times, field values/lists/ranges, ordinals (L/#), and accepts month/weekday names in the input (MON-FRI, JAN).

Round-trip property

The English is phrased so that toCron parses it back to the same expression:

toCron(toHuman(cron)) === cron

This is the round-trip oracle from the design: the readback reuses the toCron grammar rather than reinventing a cron describer, and the test suite asserts the property over every cron the forward direction can produce.

Scope

The readback prioritizes correctness and round-trip fidelity over perfect prose. English-only (no i18n). Rare composite shapes (e.g. a wildcard seconds field combined with a specific time) may not get the prettiest phrasing, though the round-trip holds for everything the forward parser generates.

Checks

• npm run typecheck, npm run lint, npm run build: clean.
• npm test: 142 tests (67 forward + 75 reverse), all green. test/to-human.test.ts round-trips every forward golden cron plus explicit readability assertions.