This sets the `catch` flag on the emitted AST when an operation/attribute has
the `[Throws]` extended attribute on it.
Additionally, constructors aren't annotated with `[Throws]` but can still throw
exceptions, so we must conservatively assume *every* constructor can throw an
error.
* webidl: Remove exact-output tests
These have not been as effective, nor as easy to write and maintain, as the
project()-based integration tests.
* tests: Move webidl tests into the webidl crate's test suite
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.
