T.P.Format: add new function formatFromFilePaths [API Change]

[tarleb]

The module Text.Pandoc.App.FormatHeuristics is removed.

[tarleb]

This is an alternative to #8693. Cc @mhwombat, who wrote that PR.

[tarleb]

I forgot to take the new function out of the Pandoc Monad; will fix that later.

[jgm]

This looks like a good improvement to me! I noted one typo.

[tarleb]

Thanks!
Large parts of lua-filters.md are now generated from the docs in the Haskell sources: the idea is that we can build an online help system without our other docs getting out of sync. That's why the pandoc.cli.repl note was in this PR: I had added the note to the Haskell sources, but forgotten to run make -B doc/lua-filters.md afterwards. The command updates that file with the docs from the sources. The repl note was added here when I ran it after adding the new from_path function.

There are still some API docs of lua-filters.md that are not generated from the sources, as the documentation system in HsLua is not powerful enough to capture the details adequately. I'm working on that.