[vim] Load configuration for the current directory

[whisperity]

Either via direnv or preferably find some relevant Vim magic.

Use case: if we are in the LLVM tree, :set shiftwidth=2, otherwise use the one in the normal init.vim.
Maybe we can put a file somewhere in the llvm/ tree (preferably at the top, like .gitignore and .envrc) that Vim can find when starting (walk up the tree from cwd...) and execute as vim commands?