[RFC] LICENSE: Change to Apache 2.0.

Since this repo doesn't include code that was previously in `neovim/neovim` (?), the "vim license" part is not required here, and the license can be changed to be the same as e.g. in `neovim/lua-client`.
Probably not, but it is included in the Apache license by default (see e.g. https://github.com/neovim/lua-client/blob/master/LICENSE).