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/backend/src/ast.rs

@@ -223,6 +223,7 @@ pub struct Struct {
     pub js_name: String,
     pub fields: Vec<StructField>,
     pub comments: Vec<String>,
+    pub is_inspectable: bool,
 }
 
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]

crates/backend/src/encode.rs

@@ -306,6 +306,7 @@ fn shared_struct<'a>(s: &'a ast::Struct, intern: &'a Interner) -> Struct<'a> {
             .map(|s| shared_struct_field(s, intern))
             .collect(),
         comments: s.comments.iter().map(|s| &**s).collect(),
+        is_inspectable: s.is_inspectable,
     }
 }