Add pi pico support

[tfry-git]

Apparently, earlier versions of the arduino-pico core were mbed-based. This appears to be no longer the case. At least not for the popular core, here: https://github.com/earlephilhower/arduino-pico . This patch adds support, again.

Only receive was actually tested.

[tfry-git]

Note: I have no idea what's supposed to have changed on line 1096 of IRTimer.hpp.

[tfry-git]

Ok, now cleaned up. Part of the problem seems to have been that I had used the online editor for my initial tinkering. But also you seem to have made a force push while I was preparing my patch.

[ArminJo]

Thanks again for your contribution.
Who does the CI and documentation work?
Who does the sending tests?
Who adapts the examples?

[tfry-git]

Who does the CI and documentation work?

Added documentation. I hope, I did not overlook anything. I have no idea, how to add CI, sorry.

Who does the sending tests?

Whoever first has a need for it? Seriously, I do not have the hardware setup for this, but no dedicated code was added, this is an arduino core, the board is fast, and nearly unrestricted WRT to pinout, so I do not really expect issues (famous last words, but still).

Who adapts the examples?

You mean PinDefintionsAndMore.h? I can do that, although I'm wondering, if the current approach isn't a bit over-explicit:
Many of the more recent boards are mostly unrestricted wrt pin usage, so it's no surprise, many different boards specify the same pinout. library.properties already provides for a warning for unsupported boards. Hence I'd suggest to remove the warning in PinDefinitionsAndMore.h, remove any #ifdefs that simply re-state these defaults, and all the defaults, silently.

Your call, of course.