Fix collab editor line break disappear issue

[MrOrz]

Fixes #550

• Declare code: true on paragraph text, which causes some commands to behave differently.
  • Proof of concept codesandbox: https://codesandbox.io/s/codemirror-vn88l4?file=/src/index.mjs:678-750
  • We keep paragraph because prosemirror doc says that at least one child node in nodes that should block content.
  • The block level serves backward compatibility
    • Existing human-edited transcripts are made of multiple paragraphs + text
    • new AI transcripts in prosemirror-schema-basic, which also defaults to one single paragraph, with other stuff that we don't need)
  • The block level would also help as we support comments and sections in the future.
• Simplify commands by replacing prosemirror-example-setup with just Enter
  • prosemirror-example-setup uses baseKeymap in prosemirror-commands
  • baseKeymap implements enter command under code blocks; we just take the effective code from there
• Remove unused parts

Acceptance criteria

• Website can load transcripts newly created by AI (one paragraph with \ns)
• Website can load human edited transcript (with multiple paragraphs)
• When the user edits content, all paragraphs stay as-is
• When the user press enter, one single \n is added, with no new paragraphs created.
• History still works

Screenshot

Can load AI generated transcript (multiple \ns)

line-break-preserved.mp4

History can still work

history-still-wrok.mp4

[coveralls]

coverage: 76.218%. remained the same when pulling c39cc79 on collab-line-break into 2b90d06 on master.