add jsEvalHandleUnknown method to handle unknown variables simillar @if does
it is easy customize the helper to use either jsEvalPure or jsEvalHandleUnknown (template author can't decide at this moment)
almost all tests from @if pass
some tests are marked as INCOMPATIBLE and they need to use {:unknown} block to give same results as @if
few tests which use non-js access to values are disabled for now