Refactor our modal dialogs to share a lot more code

[simonw]

We have a whole lot of modal dialogs now:

• The jump menu from /-/jump, plus jump_items_sql() and makeJumpSections() plugin hooks #2732
• The set column type menu from UI for setting custom column types #2675
• The insert/edit/delete modals from Insert/edit/delete UI in Datasette core, plus makeColumnField() plugin hook #2781
• The upcoming create table modal from UI for creating a table #2787
• The upcoming alter table modal from UI (and API) for altering a table #2788

They currently duplicate a whole lot of code, and there's a real risk of divergence.

Let's design and implement an abstraction for this - probably a bit of JavaScript API and a whole lot of Web Component.