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/serialization/blocks.js

@@ -45,7 +45,7 @@ const {inputTypes} = goog.require('Blockly.inputTypes');
  * }}
  * @alias Blockly.serialization.blocks.ConnectionState
  */
-var ConnectionState;
+let ConnectionState;
 exports.ConnectionState = ConnectionState;
 
 /**
@@ -67,7 +67,7 @@ exports.ConnectionState = ConnectionState;
  * }}
  * @alias Blockly.serialization.blocks.State
  */
-var State;
+let State;
 exports.State = State;
 
 /**

core/serialization/variables.js

@@ -35,7 +35,7 @@ const {Workspace} = goog.requireType('Blockly.Workspace');
  * }}
  * @alias Blockly.serialization.variables.State
  */
-var State;
+let State;
 exports.State = State;
 
 /**