Behavior of insert* commands for contenteditable=plaintext-only

See https://bugzilla.mozilla.org/show_bug.cgi?id=1291467#c17 by @masayuki-nakano

I checked the behavior in Chrome roughly now, then, I see only the following commands work, so their definitions are required:

• Handling steps of Document.execCommand("insertParagraph") (Chrome/Safari handle this command differently from pressing Enter)
• Handling steps of Document.execCommand("insertLineBreak") (LF vs <br> (Chrome vs. Safari)
• Handling steps of the commands when caret is in special elements such as <li>
• Handling steps of Document.execCommand("insertText") (including line breaks causes insertParagraph in both, but I'm not sure it's intended since the behavior looks odd)
• Handling steps of Document.execCommand("insertHTML") (especially which elements need to be handled as line breaks)

And the other commands (except undo, redo and clipboard commands) should be defined as NOOP (including not deleting Selection).

cc @howard-e

Start with the linked Bugzilla discussion and compare the listed execCommand cases for plaintext-only editing. The work is complete when the handling steps are defined for insertParagraph, insertLineBreak, insertText, insertHTML, and special elements, while the other commands are explicitly treated as no-ops as described.