Investigate replacing `proxyquire`

[datho7561]

In order for proxyquire to work, all the libraries that you want to stub need to be imported using require() instead of import ... from 'library-name'. This is annoying, because:

• the first import strategy doesn't work well with TypeScript, so none of the library functions we import this way have type checking
• the first import strategy doesn't work well with esbuild, which is why I gave up on bundling ./src/extension.ts

A possible alternative is https://github.com/testdouble/quibble