Add async/await & promise support to Bun runner

[reesericci]

This PR adds support for async/await and promises for the Bun runner.

This allows for importing libraries using dynamic imports:

const Plot = await import("@observablehq/plot")

[casperisfine]

A test case would be appreciated.

[reesericci]

How would I go about writing one? I've never written a ruby test before.

[byroot]

Have a look at the existing ones https://github.com/rails/execjs/blob/master/test/test_execjs.rb

Even with limited Ruby knowledge you should be able to figure out the structure. You just want to invoke some JS code that make use of an async function.

[reesericci]

I did some cleanup on the code and added a test case