Add .editorconfig to help developers to indent files correctly. (#7848)

EditorConfig[1] helps maintain consistent coding styles for multiple
developers working on the same project across various editors and IDEs.
It will reduce the amount of requests to reindent files, as the eidtors
directly knows the correct settings.

[1] https://editorconfig.org

[skip ci]

Co-authored-by: Sandro Knauß <hefee@debian.org>

.editorconfig

@@ -0,0 +1,13 @@
+# top-most EditorConfig file
+root = true
+
+# Unix-style newlines with a newline ending every file
+[*]
+end_of_line = lf
+insert_final_newline = true
+
+[*.{json, php, sh}]
+charset = utf-8
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true