feat: add <Del> mapping

[ivankra]

<Del> is a common mapping to delete file/folder in many tree explorers such as vim's netrw or vscode. But in nvim-tree it fails with E21: Cannot make changes, 'modifiable' is off. Let's add it to the default key mappings as an alternative to "d".

[alex-courtis]

We don't normally change default key mappings, however I reckon this should be OK. @gegoune ?

[gegoune]

@alex-courtis I won't be using it (being on MacBook), d is more vim-idiomatic anyway. I am fine with that change, shouldn't break for many users, if any at all.

[ivankra]

I think <Del> is a more universal mapping, less surprising to new users, especially anyone coming from vim's builtin netrw which has <Del>/D for delete, d=mkdir. And since there was no mapping for <Del> - it gives an error, it shouldn't really break anyone, unless doing some weird stuff with nvimtree buffers.