Add JS::Object#to_a

[gifvex]

This is a suggestion PR for a JS::Object#to_a method.

The motivation is to be able to more naturally enumerate collections from JavaScript, without having to separately access the length property and index it:

# from
collection = JS.global[:document].querySelectorAll("p")
collection[:length].to_i.times do |i|
  element = collection[i]
  # ...
end

# to
JS.global[:document].querySelectorAll("p").to_a.each do |element|
  # ...
end

Unit tests cover some of the examples on MDN's Array.from() page.

[Largo]

Edit: Turns out i was looking in the wrong place in irb.wasm!

I should have checked using:
JS::Object.new.method(:to_a).source_location

class JS::Object
def to_a
ary = []
self[:length].to_i.times do |i|
ary << self.call(:at, i).to_i
end
ary
end
end

Very useful method, but I thought that this fails for arrays with values other than numbers. I'm not sure what "as_array[_1]" does.

irb(main):002> JS.eval("return ['a', 'b', 'c']").to_a
=> [0, 0, 0]

Ok I figured out what _1 does:

"In Ruby, starting from version 2.7, _1, _2, _3, and so on are shorthand syntax for referencing the first, second, third, etc., parameters passed to a block when you don't explicitly name them. This feature is particularly useful to make the code more concise and readable when dealing with blocks that take parameters."

Clever.