Allow for js property inspection (#1876)

* Add support for #[wasm_bindgen(inspectable)]

This annotation generates a `toJSON` and `toString` implementation for
generated JavaScript classes which display all readable properties
available via the class or its getters

This is useful because wasm-bindgen classes currently serialize to
display one value named `ptr`, which does not model the properties of
the struct in Rust

This annotation addresses rustwasm/wasm-bindgen#1857

* Support console.log for inspectable attr in Nodejs

`#[wasm_bindgen(inspectable)]` now generates an implementation of
`[util.inspect.custom]` for the Node.js target only. This implementation
causes `console.log` and friends to yield the same class-style output,
but with all readable fields of the Rust struct displayed

* Reduce duplication in generated methods

Generated `toString` and `[util.inspect.custom]` methods now call
`toJSON` to reduce duplication

* Store module name in variable

crates/shared/src/lib.rs

@@ -116,6 +116,7 @@ macro_rules! shared_api {
             name: &'a str,
             fields: Vec<StructField<'a>>,
             comments: Vec<&'a str>,
+            is_inspectable: bool,
         }
 
         struct StructField<'a> {