Various cleanup commits and bug fixes

[jeanphilippegg]

Just a few commits to make the code more robust and fix some bugs I discovered:

• denote--directory-get-files would consider a directory "20240505T010101/" as a valid note.

• Renaming a file if denote-file-types is nil would throw an error. In general, we cannot assume that the file-type is non-nil as denote-file-types may be nil.

• As discussed in #284, I have reviewed denote-filetype-heuristics and we should not return org when the file type is not found. We should just return nil. I checked all the places it is used and made sure it was okay to do so. #284 can be closed.

• Most of the time, we don't actually need to use denote-file-is-note-p and instead should use denote-filename-is-note-p. This will help when a user wants to rename a new note that has not yet been saved, for example. This is not yet fully supported, but this pull request is an improvement. I have reviewed the impact of this in the places I have made the change.

[protesilaos]

I made some comments for clarity, but I already merged this. Thank you!