Fix encoding canonical names to match WHATWG spec

Changes both implementation and tests to use canonical encoding names:
- 'ibm-866' → 'ibm866' (remove hyphen)
- 'shift-jis' → 'shift_jis' (use underscore)

Per WHATWG Encoding Standard, TextDecoder.encoding should return
the canonical encoding name, not the label used to construct it.

Fixes #5663

Author: woustachemax

src/workerd/api/encoding.h

@@ -16,7 +16,7 @@ namespace workerd::api {
 // the second label is the public identifier.
 #define EW_ENCODINGS(V)                                                                            \
   V(Utf8, "utf-8")                                                                                 \
-  V(Ibm866, "ibm-866")                                                                             \
+  V(Ibm866, "ibm866")                                                                              \
   V(Iso8859_2, "iso-8859-2")                                                                       \
   V(Iso8859_3, "iso-8859-3")                                                                       \
   V(Iso8859_4, "iso-8859-4")                                                                       \
@@ -49,7 +49,7 @@ namespace workerd::api {
   V(Big5, "big5")                                                                                  \
   V(Euc_Jp, "euc-jp")                                                                              \
   V(Iso2022_Jp, "iso-2022-jp")                                                                     \
-  V(Shift_Jis, "shift-jis")                                                                        \
+  V(Shift_Jis, "shift_jis")                                                                        \
   V(Euc_Kr, "euc-kr")                                                                              \
   V(Replacement, "replacement")                                                                    \
   V(Utf16be, "utf-16be")                                                                           \

src/workerd/api/tests/encoding-test.js

@@ -401,10 +401,10 @@ export const allTheDecoders = {
       ['utf-8', 'utf-8'],
       ['utf8', 'utf-8'],
       ['x-unicode20utf8', 'utf-8'],
-      ['866', 'ibm-866'],
-      ['cp866', 'ibm-866'],
-      ['csibm866', 'ibm-866'],
-      ['ibm866', 'ibm-866'],
+      ['866', 'ibm866'],
+      ['cp866', 'ibm866'],
+      ['csibm866', 'ibm866'],
+      ['ibm866', 'ibm866'],
       ['csisolatin2', 'iso-8859-2'],
       ['iso-8859-2', 'iso-8859-2'],
       ['iso-ir-101', 'iso-8859-2'],
@@ -589,14 +589,14 @@ export const allTheDecoders = {
       ['x-euc-jp', 'euc-jp'],
       ['csiso2022jp', 'iso-2022-jp'],
       ['iso-2022-jp', 'iso-2022-jp'],
-      ['csshiftjis', 'shift-jis'],
-      ['ms932', 'shift-jis'],
-      ['ms_kanji', 'shift-jis'],
-      ['shift-jis', 'shift-jis'],
-      ['shift_jis', 'shift-jis'],
-      ['sjis', 'shift-jis'],
-      ['windows-31j', 'shift-jis'],
-      ['x-sjis', 'shift-jis'],
+      ['csshiftjis', 'shift_jis'],
+      ['ms932', 'shift_jis'],
+      ['ms_kanji', 'shift_jis'],
+      ['shift-jis', 'shift_jis'],
+      ['shift_jis', 'shift_jis'],
+      ['sjis', 'shift_jis'],
+      ['windows-31j', 'shift_jis'],
+      ['x-sjis', 'shift_jis'],
       ['cseuckr', 'euc-kr'],
       ['csksc56011987', 'euc-kr'],
       ['euc-kr', 'euc-kr'],