home-manager: add .editorconfig

Sets the basic editor settings to Home Manager style guide.

.editorconfig

@@ -0,0 +1,21 @@
+# http://editorconfig.org
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+# The JSON files contain newlines inconsistently
+[*.json]
+insert_final_newline = ignore
+
+# Makefiles always use tabs for indentation
+[Makefile]
+indent_style = tab
+
+[*.md]
+trim_trailing_whitespace = false