Refactor Registers to take Editor

This sets up a new Registers type that will allow us to expand support
for special registers. (See the child commits.)

We start simple with the regular (`Vec<String>`) registers and the
simplest special register, the black hole. In the child commits we
will expand these match arms with more special registers.

The upcoming special registers will need a few things that aren't
possible with the current Registers type in helix-core:

* Access to the `Editor`. This is only necessary when reading from
  registers, so the `&Editor` parameter is only added to
  `Registers::read`.
* Returning owned values. Registers in helix-core returns references
  to the values backed by the `Vec<String>` but future special registers
  will need to return owned values. We refactor the return value of the
  read operations to give `Cow<str>`s and iterators over those.
* Returning a `Result` for write/push functions. This will be used by
  the clipboard special registers.

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