use SPDX license specifier for plugins

While parsing the plugin API I just noticed that there is no consistent way for plugin authors to specify the license of their plugin:
```text
BSD & GPL v3+
GPL
GPL-3.0+
GPL 3.0 / fair use
GPLv3
GPL v3
GPL v3+
GPLv3 or later
InnoCraft EULA
MIT
```
It may be useful to require SPDX specifiers so it is consistent in the future:
https://spdx.org/licenses/