Simplify globalThis export

[NeilFraser]

Currently (on the goog_module branch) the Blockly.utils.global sets exports.globalThis. As a result, its users invoke:
const {globalThis} = goog.require('Blockly.utils.global');

Since this util only does one thing, wouldn't it be better to set exports, allowing its users to invoke:
const globalThis = goog.require('Blockly.utils.global');

Simplifies the API. Not a breaking change as long as we make the change before the next release.

[cpcallen]

This was in fact exactly what Blockly.utils.global did until PR #5451.

It was changed to a named export in part because the style guide says not to use default exports, but more particularly because exporting the global object (window) as the default export from a goog.module fails because (absent a call to goog.module.declareLegacyNamespace) base.js attempts to seal the exports object, and this cases an error to be thrown (at least by V8 in Chrome) when the exports object is window:

> Object.seal(globalThis)
VM103:1 Uncaught TypeError: Cannot seal
    at Function.seal (<anonymous>)
    at <anonymous>:1:8
(anonymous) @ VM103:1

(Interestingly enough, in node.js V8 will let you seal the global object, and doing so causes any subsequent var declarations in the global scope to fail with TypeError: Cannot add property <name>, object is not extensible, as one would expect.)

We could leave the declareLegacyNamespace call in this one module, or unset goog.SEAL_MODULE_EXPORTS to disable sealing more generally, but since we want to get rid of default exports anyway I went with the solution proposed in #5451 as amended by my note in #5503.

[NeilFraser]

Thanks for the explanation.