Package as Arduino library

[spacenate]

I basically just followed these instructions provided by Arduino: https://docs.arduino.cc/hacking/software/LibraryTutorial

• Moves example to examples directory
• Adds Arduino keywords.txt
  • This enables syntax highlighting inside the Arduino IDE
• Fixes a minor compilation warning in the example
• Adds include guards for Arduino.h + USBHost_t36.h
  • The first is useful in case someone is trying to use this library outside of the Arduino framework
  • The second is a helpful guard against someone trying to use this library for a (currently) unsupported board
• Calls Serial.begin() from inside the example, instead of in the library
  • The library does not use Serial, only the example does

I tested these changes with Teensyduino 1.55 (Arduino 1.8.16). After adding this library to my libraries folder:

• The example shows up in File > Examples menu as expected
• The example builds successfully for T3.6, T4.0, and T4.1
• The example fails to build for unsupported boards, with a helpful error message displayed

Resolves #1