[Tooling] Using TagHelpers in void returning methods at design time does not cause an error

[NTaylorMullen]

Describe the bug

TagHelpers aren't indicated as an error inside of a void returning method at design time. This is due to how we change rendering at design time vs. runtime.

i.e. no errors:

[NTaylorMullen]

An interesting case here will be handling things like ~/ which don't "TagHelperify" at design time but do at runtime.