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.

首先检查 issue 中确定的 repository 文件，包括 Interrupts.h，并将其许可证信息与 Arduino FAQ 进行比较。在 maintainer 或法务的指导下确定适当的许可证措辞，然后添加 repository 级别的 LICENSE 文件，并为适用的文件添加一致的许可证信息。当所有包含文件的许可证状态都得到明确说明时，即视为完成。

由索引模型根据 Issue 内容生成。