Add composite`/album/${albumID}/song/${songID}`

[RoyalIcing]

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• Tagged template string composite()
• Takes any number of elements: regex, constant string, array of strings, or generator function.
• Transforms into generator function. Requires no changes to parser.

Example:

function* AlbumID() {…}
const songIDRegex = /^\d+/;

function* GetSong() {
  const [albumID, [songID]] = yield composite`/album/${AlbumID}/song/${songIDRegex}`;
}