refactor!: Refactor cronjob controller to reduce duplication (#3421)

The cronjob controller contained a lot of essentially duplicated logic
after the addition of background events. To resolve this, I've
refactored the cronjob controller to treat cronjobs as recurring
background events. This means:

- Cronjobs and background events are now (re)scheduled and executed
using the same logic.
- Cronjobs are stored in state just like background events.
- I removed the `lastRun` property in favour of a `date` property. We
can now simply check if the current date is past the scheduled date to
know if a cronjob should be executed immediately.
   - This simplifies the logic for rescheduling cronjobs significantly.
- Cronjobs can now use ISO 8601 durations as well as the usual cron
expression.
- The logic for getting all cronjobs from all Snaps was removed, since
the full cronjobs are stored in state now.
- This works fine for new Snaps, but I need to consider how to handle
existing Snaps at the time of the cronjob controller update, either with
a migration or by re-adding some of this logic.

In addition to these changes, I've made some smaller changes to the
controller like replacing TypeScript's `private` modifier with true hash
private functions.

## Breaking changes

- The `jobs` state property was removed, cronjobs are now stored in the
`events` property as well.
- `CronjobController:schedule` now expects a `schedule` field instead of
`date`.
- Some actions were renamed to remove the `BackgroundEvent` suffix.
- `CronjobController:scheduleBackgroundEvent` ->
`CronjobController:schedule`
- `CronjobController:cancelBackgroundEvent` ->
`CronjobController:cancel`.
   - `CronjobController:getBackgroundEvents` -> `CronjobController:get`.

Author: Mrtenz

packages/snaps-controllers/coverage.json

@@ -1,6 +1,6 @@
 {
-  "branches": 95.26,
-  "functions": 98.42,
-  "lines": 98.79,
-  "statements": 98.62
+  "branches": 95.29,
+  "functions": 98.65,
+  "lines": 98.83,
+  "statements": 98.66
 }

packages/snaps-controllers/package.json

@@ -98,6 +98,7 @@
     "@xstate/fsm": "^2.0.0",
     "async-mutex": "^0.5.0",
     "concat-stream": "^2.0.0",
+    "cron-parser": "^4.5.0",
     "fast-deep-equal": "^3.1.3",
     "get-npm-tarball-url": "^2.0.3",
     "immer": "^9.0.6",