.editorconfig: add indentation rule for shell files

EditorConfig is a way to specify some basic code formatting rules
independently of an $EDITOR being used.

Add rules for bats/bash/sh files:
 - use 4 spaces for indentation (which appears to be a de-facto standard
   in this repository, see e.g. commit 86e55d0ec1acd79f7);
 - ignore everything in vendor directories.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>