Enable es-module-shims usage in web workers

[costingeana]

The purpose of this commit is to enable the usage of es-module-shims in module web workers, which at the moment are supported in Chrome based browsers and Safari > 15. As you know, in Firefox the support is still missing.

However, web workers cannot access objects like document or window. Therefore, I had to do the following changes in es-modules-shims code:

• add some simple pieces of code that check whether document/window exists before doing something with the document/window object; it's not elegant, but these checks are necessary;
• add a function that computes the baseUrl. In fact, this function was already defined in es-module-shims version 0.4.7.
• I also added a new function setImportMap which can be used to set the import map from outside (obviously). This function is useful when you start a web worker which uses es-module-shims.

It’s important to mention that in web workers, I use the es-module-shims.wasm.js output. Why? Because of the dynamicImportScript function; I’m not sure if there is a solution for the CSP version in web workers.
Also, I want to emphasize that I didn't add new tests because no new functionality was added. The existing tests cover the changes from this pull request. The usage of es-module-shims in web worker is a decision that belongs to the client code.

Below, I attach a piece of code that starts a web worker which uses es-module-shims:

// baseURL, esModuleShimsURL, aURL are provided

const workerScriptUrl = URL.createObjectURL(new Blob(
        [`importScripts('${new URL(esModuleShimsURL, baseURL).href}');importShim.setImportMap(${JSON.stringify(importShim.getImportMap())});importShim('${new URL(aURL, baseURL).href}').catch(e=>setTimeout(()=>{throw e}))`],
        { type: 'application/javascript' }))

const worker = new Worker(workerScriptUrl);

[guybedford]

Thanks this looks good!

Could you add a small documentation section for workers, explaining how this can be used? This may well be useful to others if they know about it.

See note about unifying some of the guards.

Perhaps worker CSP may be possible using import scripts of a blob? Not sure, but may well make sense to keep current CSP separation just unsupported for now.

[costingeana]

Hello @guybedford and sorry for the delay.

I applied the suggested changes and I added a small documentation section; however, I'm not sure if it's in the right place.
For now, CSP is not supported for workers. I will try your suggestion in the next days.

Thank you.

[guybedford]

Couple more doc and code comments, but I think we're there. Thanks I really like how simple this approach turned out.

I'll aim to merge in the next few days.

[guybedford]

I added the addImportMap reimplementation and rename, please just check that still works out correctly for your use cases.