Replace defx with a better file explorer/tree.

[CyberMango]

Defx is the current file tree plugin. It works fine after heavy configurations, but it still aint perfect. Remaining issues:

• tabedit from inside of defx opens the file in the defx buffer window.
• cant recursivley open already openend dirs.
• no easy way to open a dir like in an explorer.
• file open might move to different tab if file is opened there (due to drop's behavior).
• In general, requires programming too much by myself due to missing features.

should be replaced with a better plugin. Options:

• tree.nvim: supposed to be fast. might be buggy due to it being in cpp. Not yet finished.
• coc-explorer: looks good. Uses coc which is already installed. Not yet finished.

[CyberMango]

Defx has entered maintenance mode. https://github.com/kyazdani42/nvim-tree.lua has sort of become the standard now. It looks good - featureful, popular and therefore probably stable, written in lua so no external deps and less clunkiness. seems to require configurations but not as heavily as defx.