✨ Add presence support

[alecgibson]

This change adds support for the transformPresence() method that
sharedb uses.

We add support for both text0 and json0.

text0

The text0 implementation leans on the existing
transformPosition, and takes its form and tests from
rich-text.

Its shape takes the form:

{
  index: 3,
  length: 5,
}

Where:

• index is the cursor position
• length is the selection length (0 for a collapsed selection)

json0

The json0 implementation has limited functionality because of the
limitations of the json0 type itself: we handle list moves lm, but
cannot infer any information when moving objects around the tree,
because the oi and od operations are destructive.

However, it will attempt to transform embedded subtypes that support
presence.

Its shape takes the form:

{
  p: ['key', 123],
  v: {},
}

Where:

• p is the path to the client's position within the document
• v is the presence value

The presence value v can take any arbitrary value (in simple cases it
may even be omitted entirely).

The exception to this is when using subtypes, where v should take the
presence shape defined by the subtype. For example, when using text0:

{
  p: ['key'],
  v: {index: 5, length: 0},
}