handle starting and ending whitespace in song regex (#11158)

Author: riknoll

pxtblocks/fields/field_musiceditor.ts

@@ -39,7 +39,7 @@ export class FieldMusicEditor extends FieldAssetEditor<FieldMusicEditorOptions,
         let song: pxt.assets.music.Song;
 
         if (text) {
-            const match = /^\s*hex\s*`([a-fA-F0-9]+)`\s*(?:;?)\s*$/.exec(text);
+            const match = /^\s*hex\s*`\s*([a-fA-F0-9]+)\s*`\s*(?:;?)\s*$/.exec(text);
 
             if (match) {
                 song = pxt.assets.music.decodeSongFromHex(match[1]);