Allow setFragmentData without clipboardData object

[dylans]

Description

It is highly useful to get a copy of the exact selection and it is available in slate-react as e.setFragmentData. Unfortunately the type definition currently requires a DataTransfer object which is difficult to stub as this object comes from ClipboardData or drag-n-drop events. In practice, the only requirement slate-react actually needs is getData/setData. Users could provide a data object that looks like this (let me know if this should be provided as a utility method):

  const getDataTransfer = () => {
    const storage = new Map()
    return {
      getData: (key: string) => storage.get(key),
      setData: (key: string, value: string) => storage.set(key, value),
    }
  }

Issue
Fixes: Allows an easy way to get access to the slate tree in application/x-slate-fragment, 'text/html', or 'text/plain' without a copy/paste or DnD operation.

Example

The API footprint is exactly the same other than how the DataTransfer object gets passed in. Using the above method, users could do something like:

const data = ReactEditor.setFragmentData(getDataTransfer())
const fragment =  data.getData('application/x-slate-fragment')
if (fragment) {
  const decoded = decodeURIComponent(window.atob(fragment));
  const parsed = JSON.parse(decoded) as Node[];
  console.log(parsed) // see the slate tree for the selection
}

It's not 100% certain to me if setFragmentData should be async, so I've left it sync.

Context
See above. Let me know if this should work differently and I'll update accordingly.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test. (Note: I did not write a new test for this change)
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: 2711328

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[dylans]

Any chance this one can get a review? It's really just relaxing the type definition to what is actually needed within the method, and returning data.

This makes it much easier to get the slate ast from whatever is selected within the document.

The workaround currently is to clone the method in my codebase, but given how many workarounds are included I really don't want to maintain a fork. Thanks!

[timbuckley]

Completely benign type-relaxing, no need for tests IMO. Approved!

(Btw, in comments w/ code examples, it's helpful to add a lang tag to the tics, eg: )

` ` `ts 
const code = 'is highlighted in typescript'
` ` `

rendering:

const code = 'is highlighted in typescript'