Add `.to_icechunk()` method to `ManifestGroup`

[TomNicholas]

This (WIP) PR adds a to_icechunk() method directly on ManifestGroup, and refactors the .to_icechunk() virtual dataset accessor method to go via the new ManifestGroup method.

The point is

1. To further modularize the code by providing the dual of going from ManifestStore to virtual dataset, by adding the ability to go back from a virtual dataset to a ManifestStore,
2. To therefore allow users to create and persist virtual references for a single file without ever using xarray, i.e. enable #521.

After this PR we should do the same for .to_kerchunk() - at that point we could actually make xarray an optional dependency if we want to.

• [x] Closes first step of #521
• [ ] Tests added
• [ ] Tests passing
• [ ] Full type hint coverage
• [ ] Changes are documented in docs/releases.rst
• [ ] New functions/methods are listed in api.rst
• [ ] New functionality has documentation