Add support for chunked text and markdown outputs

[jonatanklosko]

See livebook-dev/livebook#2174.

[josevalim]

LGTM! I am not sure about the name, :streaming, :squash, :chunk, :merge are all good to me. Your call.

[jonatanklosko]

I am not sure about the name, :streaming, :squash, :chunk, :merge are all good to me. Your call.

Initially my idea was to use :squash and always merge with the previous text regardless its mode. But then I though that if someone renders standalone text with Kino.Text.new("yo"), we shouldn't squash into that text for whatever reason. When rendering we know if the first text should be mergeable or not. Either of :chunk, :merge or :fuse would work for me, but I like :chunk a bit more because it speaks to the type of text rather than action :)