💡 Add shortcodes for invoking different block types

[stephtr]

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 💡Indent inline text on tab key pressed #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.

[neSpecc]

Maybe related to #709

[ollietreend]

It'd also be nice to include support for pasted content. So a pasted snippet containing a line beginning # would automatically become a heading, for example.