After 79ccfb9372cb57afa569036ef39ead13abfba673, MediaWiki's `<pre>`
tags get rendered as `<pre typeof="mw:Extension/pre">` in Parsoid HTML.
MediaWiki's indent-pre syntax (block indented by a single space) is
still rendered as `<pre>` in Parsoid HTML, however.
Indent-pre is still handled by MWPreformattedNode (no changes).
Introducing MWPreNode, which will handle `<pre>` extension tags,
and MWPreDialog to change its contents (and allow converting
to MWPreformattedNode).
Pieces copied from MWGalleryNode, MWLinkNodeInspector, CommentInspector.
Possible future improvements:
* Add a specific icon for MWPreContextItem
* Avoid API roundtrips for rendering (but rendering wikitext <pre>
is not as simple as it looks)
* Consider a way to insert these other than '<pre' sequence
Bug: T159900
Change-Id: I5bc4ea6e5d893736f65ef0dd43b08c18cb1a1e85
