Add license for all files in the ArduinoCore-API repo

Some files (e.g. Interrupts.h) do not contain any license information. This makes them proprietary, so noone can copy / fork this repository without your explicit allowance. This contradicts the information "the C/C++ microcontroller libraries are under the LGPL" (see [Arduino FAQ](https://www.arduino.cc/en/Main/FAQ)). If you want to make them useable, please add license information to those files.

Additionally, please add a LICENSE file to the ArduinoCore-API repository. This would make it easier to be sure about the license status of the included files. Currently users have to check the license information in each individual file before e.g. copying / modifying / redistributing.
You could start by checking whith your lawyer if LGPL 2.1 or newer would be the best choice for the LICENSE file or if the LICENSE file has to specify the license for each file individually.

Start by reviewing the repository files identified in the issue, including Interrupts.h, and compare their license information with the Arduino FAQ. Determine the appropriate license wording with maintainer or legal guidance, then add a repository-level LICENSE file and consistent license information to the applicable files. Done means the licensing status of all included files is explicit.

Written by the indexing model from the issue text.