refactor: Be more selective in eslint ignorance (#5955)

Rather than ignoring blockly_uncompressed.js, gulpfile.js and
core/utils/global.js entirely, add suitable eslint-disable directives
to those files so that they lint cleanly (and migrate gulpfile to
use const instead of var).

core/utils/global.js

@@ -16,6 +16,8 @@
 goog.module('Blockly.utils.global');
 
 
+/* eslint-disable no-undef, no-invalid-this */
+
 /**
  * Reference to the global object.
  *