Include library to generate tones

[jcarranz97]

I would be nice to have a library that generates tones inside the pico-sdk.

A lot of people is familiar with the tone() function from Arduino which simplifies a lot the process of including a speaker or buzzer in your projects.

[lurch]

Closing, as the linked PR #1569 has been closed too.
See raspberrypi/pico-extras#70 instead.