Add a section about common memory management patterns between wasm and JS

[fitzgen]

This would be a new reference sub-section.

Things to mention:

• [ ] Dynamically allocated objects owned by JS must be manually freed by JS

• [ ] To help alleviate that pain, mention the JS equivalent of RAII:

  function withResource(f) {
      let resource = new Resource();
      try {
          return f(resource);
      } finally {
          resource.free();
      }
  }
  

  also the async version of that, with return await f(resource);

• [ ] Static globals in rust that don't need to have their lifetime managed by JS

• [ ] Mention upcoming GC weakrefs, link to tracking page, explain their relevance to cleaning up objects in wasm

• [ ] Mayyyybe mention how one could build reference counting in JS of dynamically allocated wasm objects? I don't know of anyone actually doing this, so maybe not mention it until then?

Anything else?