Belay

Features
* Added the command `clean` to the belay CLI.
* `belay clean` will delete any python files not referenced in `tools.belay.dependencies` in `pyproject.toml`.
* This cleaning action will automatically be ran at the end of `belay update`.

Bug Fixes
* Prints from device are now immediately forwarded to stdout after a newline is hit. Previously, output would only be printed after the invoked task finished executing.

Features
* Belay now features a rudimentary package manager via the CLI interface. See [the docs](https://belay.readthedocs.io/en/latest/Package%20Manager.html) for details.
* Dependencies are specified by URLs to py files in `pyproject.toml`.
* Dependencies are downloaded/updated via `belay update` and cached to `.belay-lib/`. This directory **should** be committed to project git repos.
* Dependencies and project are minified/compiled and synced to device via `belay install`.

Features
* Adds attribute `device.implementation.emitters` for detecting available emitters (`native` and `viper`).

Bug Fixes
* Fixes sync on devices without native/viper emitters available.
* Fixes sync on circuitpython devices.

Bug Fix
* Fix broken `mpy-cross`.

Changes
* Remove `lox` from dependency list.