Null-safety migration + DOM/DOW correctness fixes

[justintout]

Migrates the package to sound null safety (Dart 3) and fixes the day-of-month / day-of-week matching bugs reported by several users.

Issues addressed

• Fixes Migrate to null safety #6 — Migrate to null safety
• Fixes Cron Next & Next Relative To set the next day.  #3 — Cron next / nextRelativeTo set the next day
• Fixes Scheduling cron for day of month / weekday does not work #4 — Scheduling for day of month / weekday does not work
• Fixes Day parameter seems buggy, e. g. 0 19 7 8 * returns 2021-08-01 19:00:00 #5 — 0 19 7 8 * returns 2021-08-01 19:00:00
• Fixes Test "Cron matchers day of week we pass a test for 'the bug'" is failing #8 — "the bug" day-of-week test throws on a malformed expression
• Fixes Use max line length 80 characters #7 — Enforce 80-column formatting (dart format)

Root cause of the DOM/DOW bug

Two defects combined:

1. The POSIX day rule was not implemented. matches() always ANDed the
   day-of-month and day-of-week fields (dayOfMonthMatches && dayOfWeekMatches).
   POSIX cron treats these two fields specially: when both are restricted
   (neither is *), a time matches if either field matches; when only one
   is restricted, only that field applies. Because the code always used AND, the
   both-restricted case was wrong — this is the "cron bug" that Test "Cron matchers day of week we pass a test for 'the bug'" is failing #8's test was
   meant to cover.

2. dayOfMonthMatches evaluated the wrong field, and range parsing threw.
   The day-of-month matcher's skip (/) branch split _dayOfWeekField instead
   of _dayOfMonthField, and the hour/day-of-month/month/day-of-week matchers
   iterated a range (a-b) without a continue, then fell through to
   int.parse(value) on the range string. The net effect was that a restricted
   day-of-month such as the 7 in 0 19 7 8 * was effectively ignored, so the
   forward search stopped on the 1st of the month (2021-08-01) instead of the
   7th (2021-08-07). Likewise 0 0 1 * * / @monthly matched "tomorrow"
   rather than the 1st of the following month.

What changed

• pubspec.yaml: SDK constraint >=2.7.0 <3.0.0 → ^3.0.0; test bumped to
  ^1.24.0; version 0.1.1 → 0.2.0 (breaking change under 0.x semantics).
• lib/src/cronparse.dart: migrated to sound null safety (late final for
  fields initialized in the constructor). The five per-field matchers now
  delegate to one shared _fieldMatches helper that correctly handles
  asterisks, skips, exact values, ranges, and comma-separated sets — fixing the
  wrong-field and range fall-through bugs at their root. matches() now applies
  the POSIX OR-vs-AND day rule. Public API surface is unchanged.
• test/: added null-safety type annotations/casts; replaced the malformed
  * * *,* * SUN "the bug" expression with the valid * * 13 * SUN; added a
  regressions group with tests taken verbatim from issues Cron Next & Next Relative To set the next day.  #3, Scheduling cron for day of month / weekday does not work #4, and Day parameter seems buggy, e. g. 0 19 7 8 * returns 2021-08-01 19:00:00 #5
  (exact expressions and expected values).
• Ran dart format (80-column default) across the repository.

Verification

• dart analyze — no issues found.
• dart test — all 26 tests pass, including the new regression tests.
• dart format --set-exit-if-changed . — clean.

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