Fix evalAndReturn with comments at the bottom of a script

This is because beepbox's script:
https://cdn.jsdelivr.net/npm/[email protected]/global/beepbox_synth.min.js
has a sourceMappingURL comment at the bottom for whatever
reason, causing it to become
//# sourceMappingURL=beepbox_synth.min.js.map;return beepbox;
This is obviously going to break and might also be the case with
other libraries too, hot just beepbox.

Ported from AmpMod:
https://codeberg.org/ampmod/ampmod/commit/0daf1021bdc8e67a1f0d08b91ce500605fcaa711

Author: 8to16

src/extension-support/tw-external.js

@@ -79,7 +79,7 @@ external.blob = async url => {
 external.evalAndReturn = async (url, returnExpression) => {
     const res = await external.fetch(url);
     const text = await res.text();
-    const js = `${text};return ${returnExpression}`;
+    const js = `${text}\nreturn ${returnExpression};`;
     const fn = new Function(js);
     return fn();
 };