Adding a go version compatibility package.

Currently enables Errors.Is functionality as ErrorsIs.
Using this functionality in errors.go and unit tests.

Signed-off-by: Matthias Hanel <mh@synadia.com>

server/parser_test.go

@@ -15,7 +15,6 @@ package server
 
 import (
 	"bytes"
-	"errors"
 	"testing"
 )
 
@@ -583,7 +582,7 @@ func TestMaxControlLine(t *testing.T) {
 
 	pub := []byte("PUB foo.bar 11\r")
 	err := c.parse(pub)
-	if !errors.Is(err, ErrMaxControlLine) {
+	if !IsErr(err, ErrMaxControlLine) {
 		t.Fatalf("Expected an error parsing longer than expected control line")
 	}
 }