deps: update V8's postmortem script

This commit updates V8's postmortem metadata generation script
to support V8 7.8.

The following metadata has changed:

- v8dbg_class_JSDate__value__Object
  - The postmortem metadata generation script needed to be
    updated. No action should be required by postmortem tools.

- v8dbg_class_JSRegExp__source__Object
  - The postmortem metadata generation script needed to be
    updated. No action should be required by postmortem tools.

Author: cjihrig

deps/v8/tools/gen-postmortem-metadata.py

@@ -259,6 +259,8 @@
     'JSArrayBuffer, byte_length, size_t, kByteLengthOffset',
     'JSArrayBufferView, byte_length, size_t, kByteLengthOffset',
     'JSArrayBufferView, byte_offset, size_t, kByteOffsetOffset',
+    'JSDate, value, Object, kValueOffset',
+    'JSRegExp, source, Object, kSourceOffset',
     'JSTypedArray, external_pointer, uintptr_t, kExternalPointerOffset',
     'JSTypedArray, length, Object, kLengthOffset',
     'Map, instance_size_in_words, char, kInstanceSizeInWordsOffset',