💡 Add shortcodes for invoking different block types

1. *Describe a problem.*
When mixing different blocks, currently interaction with the toolbar is necessary. It would be nice if every block type could have a specific shortcode, with which one can invoke it.

2. *Describe the solution you'd like. Mockups are welcome.*
When an empty, new line is focused, entering "* " or "- " will convert it to a list, "# " to a heading. With that one can achieve a Markdown-like typing experience.
Since it can be extended to custom blocks, it would also solve #1163, since one could map "\t" to a custom intended block or "$ " to a math equation block.

3. *Are there any alternatives?*
Since this idea is focused on the "solution", I haven't thought much about the problem it tries to solve or other approaches.