sync-help: ensure trailing comma before inserting new help line

Multi-line field whose last property had no trailing comma got
corrupted into invalid JS:

  civiField: `${G0}.X`
  help: "...",

(JS requires the comma between properties.) The insertion path now
checks the last non-whitespace, non-comma char of the property line
just above the closing brace; if it isn't already a comma, one is
inserted before the new help line is spliced in.

scripts/sync-help-from-civi.mjs

@@ -233,12 +233,25 @@ function rewriteText(text, changes) {
     } else {
       // Multi-line: insert a new help: line just before the closing `}` line,
       // using the indent of the first property after `{`.
+      //
+      // The previous-property line may or may not end with a trailing comma
+      // (JS allows the last property to drop the comma). When it doesn't,
+      // inserting a new `help:` line below it produces `prevProp <NL> help:`
+      // which is a syntax error. So: if the line right before the closing
+      // `}` line doesn't end with a comma (ignoring trailing whitespace),
+      // append one before we splice the new line in.
       const indentM = block.match(/\{\s*\n([ \t]+)\S/);
       const indent = indentM ? indentM[1] : "  ";
       const closeIdx = f.blockEnd - 1;
       const lastNL = out.lastIndexOf("\n", closeIdx);
-      const beforeClose = out.slice(0, lastNL);
+      let beforeClose = out.slice(0, lastNL);
       const afterNL = out.slice(lastNL);
+      const prevPropTail = beforeClose.match(/([^\s,])\s*$/);
+      if (prevPropTail) {
+        // Insert a `,` right after the last non-whitespace, non-comma char.
+        const insertAt = beforeClose.length - prevPropTail[0].length + 1;
+        beforeClose = beforeClose.slice(0, insertAt) + "," + beforeClose.slice(insertAt);
+      }
       out = beforeClose + "\n" + indent + newLiteral + "," + afterNL;
     }
   }