IDL extract: exclude IDL defined in informative sections

[tidoust]

The extraction of WebIDL blocks does not yet exclude IDL blocks that appear in informative sections. Specs typically do not define IDL in informative sections, but one recent occurrence is https://w3c-fedid.github.io/FedCM/#dom-identityproviderclientmetadata-client_is_third_party_to_top_frame_origin

[tidoust]

Re-reading the code, Reffy correctly filters out IDL blocks defined in informative sections (logic is in get-code-elements.mjs).

However, in the case of FedCM, extraction leverages the IDL index at the end of the spec, which includes the IDL block. Ideally, Bikeshed would not list informative IDL blocks in the final IDL index. And/Or Reffy should always re-create the IDL index, as done for ReSpec documents for now (for some reason I don't remember...)