fix(conditionalPanel): Coerce condition result to boolean

[gadenbuie]

Fixes #4126

The key fix here is to force the result of condFunc() to be boolean so that it can be correctly compared with the current state of the conditionalPanel().

condFunc() is created by scopeExprToFunc() which was previously typed as returning a boolean value. In practice, however, this function returns an unknown value depending on the user expression. scopeExprToFunc() is sufficiently general to be useful in other places, so I relaxed the type from boolean to unknown (rather than doing the boolean coercion before returning). This also then gives a hint to users of condFunc() that they should verify the return value.