Ultimate-notion

- New: Reading and inserting comments is implemented.
- Chg: `SList` is part of the public API now, i.e. is in the `ultimate_notion` namespace.
- Fix: Added missing `link_to_page` subtype `comment_id`.
- Chg: Remove `Text` as direct import, use `text` instead.
- Chg: `.props` namespace of a page now behaves like a mapping, i.e. read-only dictionary.
- Fix: Added missing `.value` function of several `PropertyValue`s.
- Add: Page method `get_property` to allow fetching a single property without reloading the whole page.
- Fix: Resolving page properties correctly if they contain more than 25 references.
- Fix: `PropertiesEndpoint.retrieve` now actually works and either returns a single property item or a list.
- Chg: Renamed `db.fetch_all()` to `db.get_all_pages()`.
- New: Query databases with a PySpark/Polars inspired DSL, e.g. `uno.prop('Name') == 'Linus Torvalds'`, implemented.
- Fix: Database pages created with `db.create_page` are now added to the session cache.
- New: Easily activate a debug mode within the config file.
- Fix: Rollup property defined on a self-referencing relation works now.
- Doc: Added a page about querying a database.
- Chg: Renamed property types `People` to `Person` and `PhoneNumber` to `Phone`.

- New: Also use the session cache for blocks.
- New: Properties of blocks can be updated.
- Fix: `has_children` doesn't return a wrong value anymore for pages and databases.
- Fix: Several issues with blocks and duplicated but ID-equivalent objects.
- Chg: Simplified dealing with rich texts by introducing `text`, `mention` and `math`.
- Chg: Children of pages are proper pages/databases instead of `ChildPage`/`ChildDatabase`.
- Chg: Renamed `PageSchema` to `Schema`.
- Chg: Accessing the properties of a page directly returns the primitive data types.
- Chg: Completely reworked the functionality of the `Table` block for more consistency and easier usage.

- Fix: Added type `unknown` in user data to generate `UnknownUser`, issue 39.

- New: Method `page.append` to append content to a page. Creation of blocks is supported!
- New: Documentation on how to create the content of a page.
- New: More robustness with respect to Notion API changes.
- New: High-level method `session.get_block` to retrieve a single block.
- Chg: Internally, `archived` property was renamed to `in_trash`.
- Chg: Renamed `.content` to `.children` of a page for more consitency.
- Chg: Use `Property` again to consistently refer to the columns of a database.
- Chg: Make use of [pendulum](https://pendulum.eustace.io/) to represent `DateRange` values.
- Chg: Renamed `page.database` to `page.parent_db` and `page.in_db` added as methods.
- Chg: Large restructering of the code base.

- New: Introduced a configuration file under `~/.ultimate-notion/config.toml`.
- New: Added a simple Google Tasks client.
- New: Added a general sychronization task for Notion to other services.
- New: Added a specific Google Tasks synchronization task.
- Fix: A page property can be deleted by setting it to `None`.
- Chg: Reworked the testing setup to use VCR.py more efficiently and be more robust.
- Fix: Tons of fixes within blocks and general page content.
- Chg: `to_markdown()` now uses an internal implementation instead of `notion2md`.

- Chg: `RichText` is now a subtype of `str` for a more convenient & consistent usage.
- Doc: Added more documentation about using databases.
- Chg: `icon` attribute of database now returns `Emoji` instead of `str`.
- New: Allow setting the `icon` and `cover` attribute of a page.
- New: Allow setting the `title` of a non-database page.
- Fix: `created_by` and `last_edited_by` return proper `User` object.