No longer make copyright lines stringly typed

[carmenbianca]

Presently, copyright lines are stringly typed in the tool. The whole thing SPDX-FileCopyrightText: 2016, 2018--2020, 2023--present Carmen Bianca BAKKER <carmenbianca@fsfe.org> is one string.

I think it might make sense to write some kind of CopyrightLine class that can parse elements from such strings. For example, the above properties of the above example:

• copyright_prefix - SPDX-FileCopyrightText
• date_range - some DateRange object that can encode the concept of 2016, 2018--2020, and 2023--present. Or, if that all is too over-engineered, just the string 2016, 2018--2020, 2023--present
• name - Carmen Bianca BAKKER
• contact - carmenbianca@fsfe.org

If implemented well, this could heavily simplify date-range merging, which is currently spaghetti code.