Add TextDecoder support for x-user-defined encoding (fixes #6039)#6040

JosephDoUrden wants to merge 2 commits intocloudflare:maincloudflare/workerd:mainfrom

JosephDoUrden:fix/textdecoder-x-user-defined-encodingJosephDoUrden/workerd:fix/textdecoder-x-user-defined-encodingCopy head branch name to clipboard