Add initial implementation of artichoke-web gem

[lopopolo]

This gem exports Artichoke::Web::Window and Artichoke::Web::Location. Together, these enable the following program, a quine, on the playground:

# frozen_string_literal: true

require 'uri'
require 'artichoke/web'

# This program is a quine built
# using Artichoke Ruby.
#
# The quine executes Rust, Ruby, and
# JavaScript with the URL as storage.

storage = Artichoke::Web::Window.location
program = storage.hash
deserialized = URI.decode(program)

puts deserialized

This PR reorganizes the crate into modules now that main has gotten too big.

[lopopolo]

This was a kind of rubygem. See #6.