[CDF-26132] A new dev plugin for v0.7 (#2197)

# Description

As part of the plan to improve development support, we want to have a
family of `cdf dev ... ` commands.

This is just the first step of enabling the `cdf dev` app and `cdf dev
run` as a subapp.

## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Changed

- Deprecating the `run` plugin in favour of the new `dev` plugin

## templates

No changes.
