encoding/json/jsontext: add symbolic Kind constants

encoding/json/jsontext: add symbolic Kind constants

Add constants for each possible Kind value (KindNull, KindTrue, etc.).
Leave the values unchanged ('n', 't', etc.).
Update documentation to reference the symbols.

Fixes #71756

Change-Id: Ib33b2ad9ee55f6f547d9e6a1c5a7f00c8400d3d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/728420
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>