encoding: make TextDecoder handle BOM correctly

Do not accept the BOM if it comes from a different encoding, and
only discard the BOM after it has actually been read (including
when it is spread over multiple chunks in streaming mode).

Fixes: https://github.com/nodejs/node/issues/25315

PR-URL: https://github.com/nodejs/node/pull/30132


Reviewed-By: Gus Caplan <me@gus.host>