Custom Vim configs on Windows

⚠️ Important note, in order for gvim to pick the correct _vimrc location (i.e. in ~/vimfiles/_vimrc instead of the default ~/_vimrc). Add a user env variable: VIMINIT=source C:\Users\simon\vimfiles\_vimrc. VIMINIT is actually automatically executed by gvim on startup, so it's like we're actually sourcing from our prefered _vimrc file instead.