[enh] add editorconfig rules for javascript files

SearXNG has a bunch of javascript files which have an indentation of
two, but the '*' rule tells my editor to use 4 spaces.

.editorconfig

@@ -16,6 +16,9 @@ max_line_length = 119
 [*.html]
 indent_size = 4
 
+[*.js]
+indent_size = 2
+
 [*.json]
 indent_size = 4
 insert_final_newline = ignore