Compatible with JS String Builtin proposal

The JS String Builtin provides a set of functions to the wasm:js-string namespace, and it is best to wit them to fit into the component model system.

1. Standardize unmanaged core module imports in the wasm:js-string namespace
2. Host can selectively implement some js functions
3. Linker can select the required shim implementation
4. As a shared core definition, other js binding libraries can import and pass js strings without loss

There are many details to discuss, such as

• wasm:js-string is not a valid world name
• Is js-string a resource?

Start by reading the linked JS String Builtin proposal, especially the wasm:js-string API, and compare it with the component model. The issue's open questions about the namespace, resource modeling, selective host support, linker shims, and shared core definitions must be resolved before the compatibility work can be considered complete.