enable `thebe` to load in `async` and `defer` script tags

[stevejpurves]

Describe the bug

context
When I do <script async src="/thebe.js"/> or <script async src="/thebe.js"/>

expectation
I expected the library to load and intialise

bug
But instead thebe does not initialise as expected

problem
This is a problem for people as their page load is slowed down significantly

Reproduce the bug

Use <script async src="/thebe.js"/> or <script async src="/thebe.js"/>

List your environment

No response