Update indentation in editorconfig

We have .mjs files now in the regexp-generator tool. Extend the same
indentation from .js files to cover them as well.

.editorconfig

@@ -7,7 +7,7 @@ end_of_line = lf
 insert_final_newline = true
 trim_trailing_whitespace = true
 
-[{README.md,package.json,*.yml,*.sh,*.js,*.case,*.template}]
+[{README.md,package.json,*.yml,*.sh,*.js,*.mjs,*.case,*.template}]
 indent_style = space
 indent_size = 2