wsjson: Add json.Encoder vs json.Marshal benchmark

json.Encoder is 42% faster than json.Marshal thanks to the memory reuse.

goos: linux
goarch: amd64
pkg: nhooyr.io/websocket/wsjson
cpu: 12th Gen Intel(R) Core(TM) i5-1235U
BenchmarkJSON/json.Encoder-12            3517579           340.2 ns/op        24 B/op          1 allocs/op
BenchmarkJSON/json.Marshal-12            2374086           484.3 ns/op       728 B/op          2 allocs/op

Closes #409

wsjson/wsjson_test.go

+package wsjson_test
+
+import (
+	"encoding/json"
+	"io"
+	"strings"
+	"testing"
+)
+
+func BenchmarkJSON(b *testing.B) {
+	msg := []byte(strings.Repeat("1234", 128))
+	b.SetBytes(int64(len(msg)))
+	b.ReportAllocs()
+	b.Run("json.Encoder", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			json.NewEncoder(io.Discard).Encode(msg)
+		}
+	})
+	b.Run("json.Marshal", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			json.Marshal(msg)
+		}
+	})
+}