Add SPDX License Identifiers

[gtirloni]

• [ ] Add LICENSE files to this and related repositories so GitHub properly identifies the repositories' license
• [ ] Adopt SPDX License Identifier IDs instead of full license text in every file
• [ ] Add CI check

Related:

• https://www.kernel.org/doc/html/latest/process/license-rules.html#license-identifier-syntax
• https://spdx.dev/learn/handling-license-info/
• https://spdx.org/licenses/Apache-2.0.html (short identifier: Apache-2.0)
• https://reuse.software/tutorial/
• https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/licensing-a-repository#determining-the-location-of-your-license

[mnaser]

I started doing this slowly here.

https://github.com/vexxhost/atmosphere/pull/1269