Support "corrections" model in Recommendations, skip "<del>"

[tidoust]

Some specifications, published as Recommendations, incorporate proposed corrections. These corrections use <del> and <ins> to highlight parts that change between the current and the proposed future version of the spec.

Reffy happily extracts dfns, idl, links, etc. in <del> and <ins> constructs, meaning that the extracts are a merge between the current and future version of the spec. This can obviously create artificial duplicates. Reffy should rather skip <del> constructs to create extracts that represent the future version of the spec.

Examples of specs with proposed corrections:

• Web Audio API: https://webaudio.github.io/web-audio-api/#AudioContext
• WOFF 2.0: https://w3c.github.io/woff/woff2/#p1
• CSS Contain 1: https://drafts.csswg.org/css-contain-1/#contain-property (but note we actually have levels 2 and 3 in browser-specs)