Allow do blocks for keyword arguments

[LilithHafner]

sort(x, by = x -> -sqrt(x))
sort(x, lt = (x, y) -> (println("comparing $x and $y"); isless(x, y)))

could become

sort([1,2,3]) by = do x
    -sqrt(x)
end
sort([1,2,3]) lt = do x, y
    println("comparing $x and $y")
    isless(x, y)
end

Motivated by #44633

[JeffBezanson]

Very interesting idea. It requires looking ahead 2 tokens, since we need to match <identifier> = do, but I think this is currently always an error after a function call expression, so it seems do-able 😁