Add modifier to the list of built-ins

[Windvis]

Small follow-up of #1151. I forgot to uncomment that in the list 😄.

It could theoretically cause a Missing helper: modifier error if staticHelpers is enabled and no arguments are passed: {{(modifier)}} but that seems like a very weird thing to do, and the template compiler already catches this:

Assertion Failed: The modifier keyword requires at least one positional arguments