syntax: javascript: recognize also the .mjs extension

The V8 JavaScript-engine developers recommend using this extension:
  https://v8.dev/features/modules#mjs

Also, add the fairly new "JavaScript source" as magic description.

This fulfills https://savannah.gnu.org/bugs/?65334.
Requested-by: Matt Whitlock <gnu@mattwhitlock.name>

syntax/javascript.nanorc

@@ -1,6 +1,7 @@
 ## Syntax highlighting for Javascript.
 
-syntax javascript "\.js$"
+syntax javascript "\.m?js$"
+magic "JavaScript source"
 comment "//"
 
 # Declarational stuff.