code: public JBBPDslBuilder Val(final String name, final String expression) {
      final Item item = new ItemVal(
          assertTextNotNullAndTrimmedNotEmpty(name),
          assertExpressionChars(assertTextNotNullAndTrimmedNotEmpty(expression)).trim()
      );
      this.addItem(item);
      return this;
    }
docs: Add virtual field which not be read but its value will be calculated by expression
  
  @param name       name of the value, it must not be null or empty
  @param expression expression to calculate value, it must not be null or empty
  @return the builder instance, must not be null