Add deprecation around DOM / Text Renderer removal from mobiledoc-kit

[rondale-sc]

In the next major bump I believe we can remove our dependency on Domrenderer and TextRenderer here with little downsides. The standalone repositories would continue to exist as they do today this function would be only for the mobiledoc format which will return a "mobiledoc" that can be passed into the standalone Renderers to reterieve their output.

The main advantage this is that it decouples us from these renderers.

The work that'd need to happen to land this is as follows:

• [ ] remove the format switch and update the assertion to throw when anything other than "mobiledoc" is passed to this function
• [ ] Update the documentation to show examples of how to do this properly with the standalone renderers

[rondale-sc]

Was looking to remove this in another PR and cross reference. But I've found that we use these renderers to enable copy/paste functionality.

I'm looking into a way to remove these without breaking this functionality, and will update this PR soon to reflect my findings.

You can see more details around this from this commit: https://github.com/bustle/mobiledoc-kit/commit/8a1ae77a390284ac20b6df3f9a67f8132281b8fd