Read only mode

Add the ability to enable read-only mode. We need to support initialization in read-only mode and toggling read-only mode on the fly.

For each plugin in list:

Improve Tools design that can be represented as WYSIWYG
Accept readOnly boolean flag in tools constructor and save it to the class’s property.
Separate event bindings and contenteditable attributes activating to the toggleReadonly(status) method. Call this method with true status on the render in read-only mode. Also, this method can be called with true/false statuses by editorjs core — it should bind/unbind events corresponded by status.

Working with editor.js core:

Add readOnly option to the Editor Config
Make readonly.ts module that will store readOnlyEnabled property and have toggleReadonly(status) public method.
toggleReadonly(status) method of readonly.ts module will call toggleReadonly(status) methods of each editor module implemented this method.
Implement toggleReadonly(status) method in modules that requires some readonly toggling logic: UI, Toolbar, InlineToolbar, Paste, Saver, Shortcuts (for disabling cmd+x), BlockSettings, BlockEvents, BlockManager, DragNdrop, ModificationObserver, API. This method will do two things: bind/unbind events and make/remove dom elements.
If some module makes own ui and bind events in prepare method (for example, UI, Toolbar): move events binding and ui making in toggleReadonly() method of this module. Then call toggleReadonly() in prepare.
Pass readOnly option to Tools constructor on render
If some of Tools does not provide read-only functionality (can be detected by toggleReadonly () implemented or not), throw an error on trying to enable read-only mode.
add toggleReadonly() method to the public API. This method will call toggleReadonly(status) method of readonly.ts module internally.