[MC][WebAssembly] support strings as import modules and names

E.g. importing a function can looks like this:
```gas
.import_module foo, some_module_name
.import_name foo, some_function_name
.functype foo () -> ()
```

However, Wasm actually supports strings here and not simply identifiers. This has come up while implementing the [ESM integration proposal](https://github.com/WebAssembly/esm-integration), where specifying paths like `./path_to_js_file.js` as the module name is necessary.

This should be quite simple to implement by supporting strings as well in that position.