Smarter extend_comment_on_newline

[injust]

The extend_comment_on_newline default of true was surprising to me, especially in Python when I often want to write a one-line comment, followed by code.

If I disable extend_comment_on_newline, then I can no longer press Enter to split a comment into two lines.

In VS Code, pressing Enter will extend the line iff the cursor is not at the end of the line. I don't think this is even configurable. IMO this is the least surprising behaviour and is a better default than the current behaviour in Zed.