Add an option to load ENV synchronous

[jiz4oh]

I have many vim plugins and some functions rely on the ENV. And those functions are also usually triggered at BufEnter and DirChanged. But those functions are work as unexpected due to the direnv load ENV asynchronous.

Could we consider add an option that let users to load ENV synchronous? I think it's better to have a performance degradation than to behave incorrectly.

I'd like to open a pr if you guys like it.

[jiz4oh]

related https://github.com/tpope/vim-bundler/issues/59#issuecomment-2108462453