Use spec references for VARC once available

Once a published VARC spec exists (opentype and/or ISO OFF) find all references to github.com/harfbuzz/boring-expansion-spec and update to include a reference to the authoritative spec. The be-spec ref can be retained if it is still useful, such as because it's terser and provides code examples.

Search the repository for references to github.com/harfbuzz/boring-expansion-spec. Once a published VARC specification exists, identify the authoritative OpenType or ISO OFF reference and update those references, retaining the existing link where it remains useful for brevity or examples.

Written by the indexing model from the issue text.