html: add NodeType.String() method

Uses stringer to add String method to html.NodeType.

Fixes golang/go#77006

Change-Id: I8120215f859f4c3e6f17757e106cff3936bfa1ba
GitHub-Last-Rev: 2082ce0
GitHub-Pull-Request: #244
Reviewed-on: https://go-review.googlesource.com/c/net/+/738100
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: earthboundkid

html/node.go

@@ -11,6 +11,7 @@ import (
 // A NodeType is the type of a Node.
 type NodeType uint32
 
+//go:generate stringer -type NodeType
 const (
 	ErrorNode NodeType = iota
 	TextNode