Add a way to mark a method as deprecated in inline RBS

For editor support in the Ruby LSP, it'd be great if this were standardized by rbs. See also https://github.com/Shopify/ruby-lsp/issues/1773

I see that there are deprecation signatures in RBS syntax that are generally standardized: https://github.com/ruby/rbs/blob/031f319726752e74499fc7036c9593a3b5d87ed0/sig/definition.rbs#L100

But is there a way to do this in an inline RBS signature? Should there be? Should we consider a @deprecated tag at the start of a comment line to be the standard way to do this, to match YARD/JSDoc and all the gems that already use this syntax?

class Foo
  # @deprecated Use #new_thing instead.
  #: -> String
  def old_thing
    return 'bad'
  end

  #: -> String
  def new_thing
    return 'better'
  end
end

I see that there is one example of an inline comment for a deprecated method, in the tests, but I don't think it technically means anything here.

https://github.com/ruby/rbs/blob/031f319726752e74499fc7036c9593a3b5d87ed0/test/rbs/inline_parser_test.rb#L1305-L1306

I am very much looking forward to Inline RBS! :)

Start by reading sig/definition.rbs around the deprecated signature and the inline deprecation example in test/rbs/inline_parser_test.rb. Compare what the existing inline parser treats as meaningful with the standardized RBS syntax. Done means the project has a decided, tested way to represent deprecation in inline RBS, if the proposal is accepted.