A single #EXT-X-DEFINE variable declaration (RFC 8216bis §4.4.2.3).
Unlike every other §4.4.2 tag, EXT-X-DEFINE MAY appear more than once
per Playlist, so it is carried as a Vec on both MediaPlaylist and
MasterPlaylist rather than a single field.
Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.
IMPORT form — imports a Variable of the same name from the parent
Multivariant Playlist. The spec says this MUST NOT occur in a
MasterPlaylist (Multivariant Playlist) — only in a
MediaPlaylist loaded from one — but that is a cross-file
constraint this single-playlist parser cannot check, so it is not
enforced here (see the module docs on this crate’s general
MUST-constraint leniency).