Migrate core.global.js to named exports (#5451)

This is part of #5153 but is being prioritised because we want remove
the declareLegacyNamespace calls from the core/utils/*.js modules and
then reexport them explicitly via utils.js, and it turns out that
doing so results in the exports object of this module being passed to
Object.freeze - which fails on the global object, which can't be made
non-extensible!

The new name chosen for the former default export is globalThis, since
it is intended to have the same value as the global variable of that
name; see:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis

generators/javascript.js

@@ -42,7 +42,7 @@ Blockly.JavaScript.addReservedWords(
     // Magic variable.
     'arguments,' +
     // Everything in the current environment (835 items in Chrome, 104 in Node).
-    Object.getOwnPropertyNames(Blockly.utils.global).join(','));
+    Object.getOwnPropertyNames(Blockly.utils.global.globalThis).join(','));
 
 /**
  * Order of operation ENUMs.