The Web IDL conversion to `BufferSource` and similar types shouldn't
check whether the buffer is detached.
In the case of `TextDecoder`, our implementation would still throw after
the Web IDL conversions because we're creating a new `Uint8Array` from
the buffer source's buffer, which throws if it's detached. This change
also fixes this bug.
The implementation of `TextDecoder` had a bug where it was copying the
input data in every case. This change removes that copy in
non-`SharedArrayBuffer` cases.
Since passing a shared buffer source to Rust would fail, this copy of
the input data was making `TextDecoder` work in cases where the input
is shared. In order to avoid a breaking change, the copy is retained in
those cases.
