Add Support for Non-Symbol Keyowrds

(Originally posted as a discussion, #1609, I think this is better as an issue)

While working on Kernel#system, I came across a limitation of RBS: it only lets you specify symbol keyword arguments, and not any other type.

Normal keyword arguments in Ruby are passed via key: value, such as gets(chomp: true). However, this syntax is actually sugar for gets(:chomp => true), and Ruby is capable of accepting other values for keys.

Take this contrived example:

def add_one(**kw)
  raise ArgumentError, "missing `0`" unless kw.key?(0)
  raise ArgumentError, "unknown keywords: #{kw.keys - [0]}" unless kw.keys == [0]
  kw[0] + 1
end
p add_one(0 => 3) #=> 4

Or this slightly-less-contrived example:

def add_exponentiated_numbers(**kwargs)
  sum = 0
  kwargs.each do |key, value|
    sum += key ** value
  end
end

puts add_exponentiated_numbers 2=>3, 4=>5, 6=>7 #=> 280968

There's no real way to typecheck this currently in RBS, as RBS only supports symbols for hash keys. It does have a **rest parameter, but that expects symbols as hash keys as well.

I propose we add two new pieces of syntax to function declarations:

• <literal> => <type>, such as def add_one: (0 => Integer).
• **<type> => <type>, such as def add_exponentiated_numbers: (**Integer => Integer).

Notably, the ** variant isn't equivalent (nor replaces) the current **<type> <variable> syntax. For example, a simplified Kernel#system (which can take integers for hash keys, which correspond to hash descriptors) could be defined as:

def system: (String cmd, **Integer => Integer, **untyped other_options) -> bool

No implementation files or tests are named. Start by reviewing RBS's existing keyword and **rest-parameter syntax, then compare the proposed literal-key and typed-key forms with the Kernel#system and add_exponentiated_numbers examples. Done means valid signatures can express non-symbol keyword keys without replacing existing syntax, with coverage for the examples.