.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 86e55d0ec1);
 - ignore everything in vendor directories.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 5c5389dddd)
Signed-off-by: Paul Holzinger <pholzing@redhat.com>

.editorconfig

@@ -0,0 +1,13 @@
+# See https://EditorConfig.org
+
+# This is a top-most EditorConfig file.
+root = true
+
+# Ignore any "vendor" directories.
+[**/vendor/**]
+ignore = true
+
+# Bash, bats, and sh files use 4 spaces for indentation.
+[*.{bash,bats,sh}]
+indent_style = space
+indent_size = 4