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/cli-support/src/webidl/mod.rs

@@ -256,6 +256,8 @@ pub struct AuxStruct {
     pub name: String,
     /// The copied Rust comments to forward to JS
     pub comments: String,
+    /// Whether to generate helper methods for inspecting the class
+    pub is_inspectable: bool,
 }
 
 /// All possible types of imports that can be imported by a wasm module.
@@ -1238,6 +1240,7 @@ impl<'a> Context<'a> {
         let aux = AuxStruct {
             name: struct_.name.to_string(),
             comments: concatenate_comments(&struct_.comments),
+            is_inspectable: struct_.is_inspectable,
         };
         self.aux.structs.push(aux);