The streaming decoder previously selected an encoding only from a byte-order
mark or the UTF sniff, so its single-byte and CJK decode paths were latent. It
now reads the encoding from the XML declaration (the libxml2 behavior) when
there is no BOM, activating those paths over an incremental byte source. Add
the streaming form of ISO-2022-JP, carrying the stateful mode across reads
(extracted to its own file to stay under the type-body cap). A test asserts
streaming agrees with whole-buffer decode for a single-byte (ISO-8859-1), a
CJK (Shift_JIS), and an ISO-2022-JP document.
Part of #108, #105
Add the WHATWG iso-2022-jp decoder, the first stateful encoding: escape
sequences switch between ASCII, JIS X 0201 Roman (yen, overline), JIS X 0201
katakana, and the JIS X 0208 plane, resolved through the shared jis0208 table
(no new data). Tested across all four modes, with the JIS X 0208 kanji
cross-checked against the EUC-JP decoder (same table, same pointer). EUC-TW and
the streaming form of ISO-2022-JP remain.
Part of #108, #105
