Fix indentation and paragraph-fill inside comments

[mattiase]

Inside comments, explicit indentation as well as the automatic indentation with electric-indent-mode active (just pressing RET, in Emacs 28) now use the indentation of the previous nonempty line, even when that line is on the same line as the initial (*.

Filling paragraphs with M-q inside a comment now only fills a single paragraph instead of the entire comment. On an empty line, the preceding paragraph is used.

Inside doc comments, lines starting with @-tags (like @param) start a paragraph, and subsequent lines are indented to the text after the tag.

Fixes #213.

[Chris00]

May you please rebase it on master?

[mattiase]

Rebased!

[mattiase]

(re)²based!