basic file drag and drop support

[unambient]

this pr adds a really basic, but functioning, implementation of drag and drop support.

i worked on this during the period of free time that i currently have, and because of that the UX isn't really the best (but also firefox seemingly doesn't want dropEffect to work? maybe i'm just dumb and was misusing it, either way it's fine)

i intend on working on this slightly more in the future if i get the chance, but i would like the code to exist in the repository just on the off-chance that i never finish polishing it.

[unambient]

i forgot to mention it in the source, but the reason for hover_count's existence is because the dragevent and dragleave behave really weirdly with children in the parent div (hovering over a child calls dragleave). so it's just a really hacky "fix" to make it so that the border outline doesn't randomly disappear.

i took the method from here, and there are probably definitely better ways of doing it, but oh well :)