Add boolean conversion to ? suffixed predicate method

[kateinoigakukun]

This feature satisfies the scenario shown in #247 in a Ruby way. The form of the shorthand looks much Rubyish and smarter than == JS::True, also it brings minimum complexity.
e.g.

searchParams = JS.global[:URLSearchParams].new(JS.global[:location][:search])
# "URLSearchParams.has" returns Boolean in JS,
# and '?' indicates the returned JS boolean is converted to Ruby boolean
if searchParams.has?('q')
  # ...
end

This idea itself is suggested by @znz. Thanks!