chore: use GitHub Actions.

- use GitHub Actions instead of TravisCI
- updates golangci-lint to v1.36.0 and applies my rules

interp/interp_file_test.go

@@ -37,6 +37,8 @@ func TestFile(t *testing.T) {
 }
 
 func runCheck(t *testing.T, p string) {
+	t.Helper()
+
 	wanted, goPath, errWanted := wantedFromComment(p)
 	if wanted == "" {
 		t.Skip(p, "has no comment 'Output:' or 'Error:'")