Task list support

[BaldissaraMatheus]

Is your feature request related to a problem? Please describe. Currently there's no task list support in stacks-editor, as described in GFM dialect. Also, when typing [ ] in markdown mode, then switching to rich text mode, then going back to markdown mode, the text is changed to \[ \].

Describe the solution you'd like Add checkbox tool that can be accessed through the rich text mode toolbar and brackets in markdown mode.

Describe alternatives you've considered It seems like the editor has support for custom plugins, but I couldn't find any documentation on how to implement one. If it's not possible to add this feature, I'd ask for any orientation on how to accomplish that by creating a custom plugin for it

Additional context The editor is being used in Tasks.md and this feature is being requested for different users