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
  and ~/.config/vim/after/syntax/python.vim
- Add `vim` artifacts in ~/.config/git/ignore

.config/vim/README.md

@@ -0,0 +1,11 @@
+# `vim`-related Configurations
+
+This folder contains
+    the [`vimrc`](./vimrc),
+    loaded via `$VIMINIT` set in [`~/.config/shell/env`](../shell/env).
+
+[`after/`](./after/) holds
+    [filetype](./after/ftplugin/)- and
+    [syntax](./after/syntax/)-specific overrides.
+
+[`spell/`](./spell/) is a placeholder for downloaded and untracked spell files.