experimental: Enter support

[istarkov]

Description

ref #4595

• Enter on block creates new paragraph, text or h1 block if any exists in templates
• New text added through / command is fully selected
• If after command block is empty it's replaced with another block
• esc now closes the / menu
• backspace on empty now deletes block

Play here

https://p-0b40acbe-34df-4475-8f52-120d5b97da4e-dot-enter.development.webstudio.is/?mode=content

• - Bug (Next PR)

to reproduce `Enter`, `/` select Heading, type `blabla`, then `/`, then `Enter`

Steps for reproduction

1. click button
2. expect xyz

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 0000)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file