This is branched off of #76467, and is related to #75431.

encoding/json/jsontext.Decoder.PeekKind returns KindInvalid (0) if the next read will return an error.

I propose that PeekKind distinguish between EOF (at end of input, input is syntactically valid, no errors encountered) and errors (I/O error, syntax error, etc.). Since we've rejected PeekKind returning an error, we could do this with an additional Kind:

const (
  KindEOF Kind = 1 // returned by PeekKind at syntactically-valid EOF
)