feat: add controls provider to render controls outside slate context

[z4o4z]

Description
This pull request adds a new ToolbarProvider component to render controls/toolbar outside the slate context.

Example

<>
      <ToolbarProvider editor={editor}>
        <Toolbar style={{ marginBottom: 20 }}>
          <MarkButton format="bold" icon="format_bold" />
          <MarkButton format="italic" icon="format_italic" />
          <MarkButton format="underline" icon="format_underlined" />
          <MarkButton format="code" icon="code" />
          <BlockButton format="heading-one" icon="looks_one" />
          <BlockButton format="heading-two" icon="looks_two" />
          <BlockButton format="block-quote" icon="format_quote" />
          <BlockButton format="numbered-list" icon="format_list_numbered" />
          <BlockButton format="bulleted-list" icon="format_list_bulleted" />
          <BlockButton format="left" icon="format_align_left" />
          <BlockButton format="center" icon="format_align_center" />
          <BlockButton format="right" icon="format_align_right" />
          <BlockButton format="justify" icon="format_align_justify" />
        </Toolbar>
      </ControlsProvider>

      <Slate editor={editor} initialValue={initialValue}>
        <Editable
          renderElement={renderElement}
          renderLeaf={renderLeaf}
          placeholder="Enter some rich text…"
          spellCheck
          autoFocus
          onKeyDown={event => {
            for (const hotkey in HOTKEYS) {
              if (isHotkey(hotkey, event as any)) {
                event.preventDefault()
                const mark = HOTKEYS[hotkey]
                toggleMark(editor, mark)
              }
            }
          }}
        />
      </Slate>
    </>

Context

• Moved most of the slate component logic into a provider component to reuse it in the toolbar-provider.
• toolbar-provider is a simpler version of slate, it doesn't mutate the editor since it must be mutated in the slate component only.
• changed EDITOR_TO_ON_CHANGE map to be a map of sets, this way we can have multiple listeners for the same editor

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• 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: 49a0373

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	Minor

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[dylans]

I'm not opposed to this change, but I've been sitting on it a bit as it requires changing a few APIs to support it and I'm not yet sure what consequences that might introduce to others.

[z4o4z]

hm, what kind of APIs does the PR change? The only thing I can see is EDITOR_TO_ON_CHANGE, but it's internal and used in only one place, so shouldn't be any problems, isn't it?