chore: replace more uses of var with const and let (#5628)

* chore: fix uses of var in core/block_dragger

* chore: fix uses of var in core/extensions.js

* chore: fix uses of var in core/field_multilineinput.js

* chore: fix uses of var in assorted core files

* chore: fix uses of var in node test runner and playground screenshot code

* fix: undefined return from measureFontMetrics

* fix: violations of no-const-assign

* chore: only one variable declaration per line

core/utils/dom.js

@@ -413,8 +413,11 @@ const measureFontMetrics = function(text, fontSize, fontWeight, fontFamily) {
   div.appendChild(block);
 
   document.body.appendChild(div);
+  const result = {
+    height: 0,
+    baseline: 0,
+  };
   try {
-    var result = {};
     div.style.alignItems = 'baseline';
     result.baseline = block.offsetTop - span.offsetTop;
     div.style.alignItems = 'flex-end';