[Feature Request]: remove {shiny} from {teal.transform}

[m7pr]

Only 2 functions from {shiny} are used in {teal.transform} in code:

• shiny::validate
• shiny::ns.sep

I reckon we can substitute shiny::validate by {shinyvalidate} package equivalents. This package is already in Imports. I need help figuring out how we could substitute shiny::ns.sep. Other than that {shiny} can go to Suggests as it's only used in examples besides the two functions I mentioned before.

Part of #194