feat:`content_render()` and `content_restart()` implementations

[toph-allen]

Intent

Add content_render(content) and content_restart(content) functions.

Approach

• content_render() takes a piece of rendered content and triggers a render via the variants API. It returns a ContentTask object. If called on non-rendered content, raises an error.
• content_restart() takes a piece of interactive content and sets then unsets an environment variable. If called on non-interactive content, raises an error.
• Added a default_variant property to the Content class, an active binding that returns its default variant.
• Added two active bindings (a.k.a. dynamic properties) to Content: is_interactive and is_rendered, both booleans that are computed from app_mode.

Added tests for all behaviors. I'm still feeling out testing this library; I feel like I'm used to more fully-specified unit tests rather than the more e2e style tests here.

[tdstein]

It appears the CI failures are due to missing documentation. Once those are added, I think this is ready for final review.

[toph-allen]

@tdstein I'm gonna address a few more comments before I merge; thanks for the approval!