Add calendar platform to Habitica integration

[tr4nt0r]

Proposed change

This adds the calendar platform to the Habitica integration and adds three calendar entities.

To-do's:

Provides due dates of to-dos as a calendar. Due dates are all-day events.

Dailies

Provides a calendar listing with all dailies for today and all future recurrences of dailies. Dailies for today will only be listed as long as they were not completed. The state of the calendar entity will be on as long as there is still tasks to do for the day and off when all is done.

Reminders

~~Habitica dailies and to-dos allow setting reminders, these will be listed in this calendar, displaying the name of the event the reminder belongs to. Reminders don't have a duration, but the calendar status will show reminders as active for up to 1hr, unless another reminder goes off.~~

Removed for now, needs some more tweaking, will be added in separate PR

Type of change

• [ ] Dependency upgrade
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New integration (thank you!)
• [x] New feature (which adds functionality to an existing integration)
• [ ] Deprecation (breaking change to happen in the future)
• [ ] Breaking change (fix/feature causing existing functionality to break)
• [ ] Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request: https://github.com/home-assistant/home-assistant.io/pull/35206

Checklist

• [x] The code change is tested and works locally.
• [x] Local tests pass. Your PR cannot be merged unless tests pass
• [x] There is no commented out code in this PR.
• [x] I have followed the development checklist
• [x] I have followed the perfect PR recommendations
• [x] The code has been formatted using Ruff (ruff format homeassistant tests)
• [ ] Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• [x] Documentation added/updated for www.home-assistant.io

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  Updated and included derived files by running: python3 -m script.hassfest.
• [ ] New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
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