Add compile-time awareness of use sites for DEPRECATED things.

[jemc]

It's become common practice for me when I do "rolling breaking changes" in Savi code to mark the old functions/types/etc as being DEPRECATED, using an annotation like this:

  :: DEPRECATED: Use `ByteStream.Pair.new` to create a reader and writer pair.
  :new to_reader(reader ByteStream.Reader.Private)
    @_target = ByteStream.Sink.ToReader.new(reader)

To make these annotations useful, we want to have the compiler be aware of them and expose features related to them, including:

• A mode for building programs with savi that will include a pass checking for deprecated use sites, that will print any deprecated use sites and fail with a nonzero exit code if any were found.
• LSP diagnostic messages (with yellow squiggly underlines) for deprecated use sites