[FEATURE] Stacked templates

[esmeetewinkel]

What?
Ability to launch and display stacked templates, with small top offset from parent template.

Figma

Related to #2347 (but doesn't replace it)

[chrismclarke]

@jfmcquade
As a first approach would likely recommend:

• Create stack_navigation service that registers action handlers for nav_stack push and nav_stack pop actions.
• Add support for pushing new template on top of existing content as part of stack. Support stack multiple levels, layered offset
• Add support for popping top level from stack

Would also be good whilst working on these to setup a decent testing environment (as more complex behaviour will need to be built on top). Ideally component spec, service spec and debug sheet.

Once an initial PR is merged can move onto more complex tasks

• Navigation within a stack
• Handling completed/uncompleted actions triggered within a stack (and how to author from nav_stack push action)
• Popups within a stack
• Custom stack styling (e.g. as an alternative to full-screen popups or regular popups)