Versions screen -- switch between text and source

[saracarl]

So our versions screen, or at least parts of it, is going to start playing a bigger part in some of the upcoming AI driven features. A first step of that is to support 2 different modes in the version comparison screen:

1. The existing versions screen (fixed per #4288) that shows formatted text, with links and bolds/italics/etc. This should come from the page's transcription field.
2. A "source text" version comparison that instead of formatting the page, shows the tags the transcriber entered for both text formating and wiki links. So a lines might look like:

China GrovE [[Mississippi--Pike County|Pike Co]]
May 10<sup>th</sup> 1876

and show differences in the mark up as well was the actual text. (i.e. make it easier to see when a link changes, which right now requires hovering and looking at the target URL of the link.) This come from the xml_transcription field of the page.

We'll also want to do this for translation versions. Those come from source_translation and xml_translation. I believe those are just stacked on top of each other in the versions screen -- that's fine to keep the same way, but just toggle together with the transcription.

Here's what I think the UI could look like (but happy to have other suggestions if you have a better idea), with one or two buttons next to the paired side by side/over under icons: But maybe a paired "source" and "document" icon like we have paired side by side/over under icons?

The source/code icon we like is this one: https://fonts.google.com/icons?selected=Material+Symbols+Outlined:code:FILL@0;wght@400;GRAD@0;opsz@24&icon.query=html&icon.size=24&icon.color=%235f6368 And for the document icon: https://fonts.google.com/icons?selected=Material+Symbols+Outlined:article:FILL@0;wght@400;GRAD@0;opsz@24&icon.query=document&icon.size=24&icon.color=%235f6368