Replace CapabilityValueOrRange::ValueUnion with an std::variant, and generate its encode/decode methods
<rdar://problem/116038461>
Pull request: https://github.com/WebKit/WebKit/pull/18210
Retitle: Replace CapabilityValueOrRange with CapabilityRange
Committed 268487@main (f685afa11aca): <https://commits.webkit.org/268487@main> Reviewed commits have been landed. Closing PR #18210 and removing active labels.