Add SPDX LicenseRef-.* for PKGBUILD's license

[txtsd]

Which means SPDX license expressions, and LicenseRef-* for custom licenses, are possible.

Reference:
https://spdx.github.io/spdx-spec/v3.0.1/annexes/spdx-license-expressions/#composite-license-expressions
https://spdx.org/licenses/exceptions-index.html

[Freed-Wu]

#19 (comment)

It should support SPDX. I heard LicenseRef-* firstly. Sounds like all string match LicenseRef-.* are legal?

[txtsd]

Correct!

[txtsd]

But also SPDX expressions like MIT OR GPL-3.0-or-later WITH GPL-3.0-linking-source-exception are valid.

EDIT: along with parentheses

[Freed-Wu]

SPDX expression looks a little complex. Now this project just use regex to match some legal string.