Introduce the wasm-bindgen-webidl frontend

This is still a work in progress. Parse WebIDL source text and convert it into
wasm-bindgen AST, so that we can automatically emit bindings for the types and
functions described in the WebIDL.

crates/webidl/tests/all/simple.rs

@@ -0,0 +1,3 @@
+use super::backend;
+
+assert_parse!(empty, "", backend::ast::Program::default());