lang: Add a resource-specific elvis operator

This allows you to omit a resource parameter programmatically, and
avoids the need of an `undef` or `nil` in our language, which would
contribute to programming errors, crashes, and overall reduced safety.

lang/lexparse_test.go

@@ -122,6 +122,20 @@ func TestLexParse0(t *testing.T) {
 			//exp: ???, // FIXME: add the expected AST
 		})
 	}
+	{
+		values = append(values, test{
+			name: "one res with elvis",
+			code: `
+			test "t1" {
+				int16 => true ?: 42, # elvis operator
+				int32 => 42,
+				stringptr => false ?: "", # missing is not ""
+			}
+			`,
+			fail: false,
+			//exp: ???, // FIXME: add the expected AST
+		})
+	}
 	{
 		// TODO: skip trailing comma requirement on one-liners
 		values = append(values, test{