Add configuration for vim

- Add ~/.config/vim/vimrc
  + Configure basic stuff
  + Activate spell checks
  + Activate syntax highlighting and ruler
  + Show whitespace characters
  + Add various snippets for mouse handling,
    toggling line numbers, and search
  + Set `vim`-related directories inside
   ~/.config/vim and ~/.local/state/vim
- Integrate extra settings for Python files
  in ~/.config/vim/after/ftplugin/python.vim
- Add `vim` artifacts in ~/.config/git/ignore

.config/shell/env

@@ -22,9 +22,11 @@ export DOTFILES_DIR="$XDG_DATA_HOME/dotfiles"  # also set in ~/.local/bin/instal
 
 # Generic shell configs
 
+export EDITOR=vim
 export GPG_TTY=$(tty)
 export PAGER="less --chop-long-lines --ignore-case --LONG-PROMPT --no-init --status-column --quit-if-one-screen"
 export TZ="Europe/Berlin"
+export VISUAL=$EDITOR
 
 export LANG=en_US.UTF-8
 export LC_TIME=en_DK.UTF-8         # ISO dates, 24h clock, Monday first
@@ -39,3 +41,4 @@ export LC_TELEPHONE=de_DE.UTF-8
 # Move common tools' config and cache files into XDG directories
 
 export LESSHISTFILE="$XDG_STATE_HOME/less/history"
+export VIMINIT='let $MYVIMRC="'"$XDG_CONFIG_HOME"'/vim/vimrc" | source $MYVIMRC'