(#7762) In DSL allow UNLESS as replacement for IF NOT

Add support to DSL for UNLESS.
Add support for two cases of UNLESS:

unless <condition> { <statements> }
unless <condition> { }

Proxy UNLESS in AST to IF, wrapping condition an (additional) NOT

Add tests to ensure that AST::Not gets added tree in "unless false {
$var = 1 }"
Add tests to ensure that no exception raised for "unless false { }"