template tag for updating cells

[tonyfast]

currently we produce all of the dead pixels on the serve to provide the initial readable view of the notebook. someone may feel comfortable in the space we've provided and reach out for an external resource. this pull request starts this lift by providing a template for cells in a specific layout.

these reason to implement this is experiment with the screen reader experience when loading new content. we are not interested in a standalone application, but a reference impl. some questions we would like to know are:

• what should aria-live provide for a good upload experience?
• do we introduce our first meter or progress bar?
• do we announce errors? out of orderedness?

[github-actions]

A preview fo the exported examples will be rendered at
https://deathbeds.github.io/nbconvert-a11y/branch/template-tag-upload/exports/html/lorenz-executed-a11y.html

[tonyfast]

current the any_cell block is scoped wrong and inheriting the wrong format. the templates will need work to produce a template from the tempaltes we are already using.

[tonyfast]

features like this will allow to understand the a11y pros/cons of shadow dom and web components. hint scope is a problem