Cleanup copyright and licensing information and make the project REUSE compliant

[mknejp]

Make the project REUSE compliant and in doing so cleanup copyright
and license handling.

This consists of:

• Creating an AUTHORS.txt file listing the "dotVariant Authors" and
  embedding it in the dotVariant package. Don't rely on version control
  history as it might be incomplete, incorrect, or just not present (by
  making a plain copy of the project files).

• Changing all the existing file headers of C# files to:

  // SPDX-FileCopyrightText: 2021 The dotVariant Authors (see AUTHORS.txt)
  //
  // SPDX-License-Identifier: BSL-1.0

• Adding the following to simple configuration files, like .gitignore
  and similar. As those are usually not considered copyrightable we use
  the CC0-1.0 license for these files, putting them in the public
  domain.

  SPDX-FileCopyrightText: 2021 The dotVariant Authors (see AUTHORS.txt)
  
  SPDX-License-Identifier: CC0-1.0
  

• Adding entries to ./reuse/dep5 for files which are mostly
  tool-generated or where maintaining a file header is difficult (or
  even impossible), or which can no longer be parsed by tools if a
  copyright header is present. They are licensed under CC0-1.0.

• Adding entries to ./reuse/dep5 for the *.out.cs sample files,
  which are tool-generated and thus have their copyright header removed
  (which conveniently also simplifies the testing procedure).
  They are licensed under CC0-1.0.

• Automating the process of embedding licenses of third-party packages
  inside the dotVariant package. Use dotnet-project-licenses to
  gather the license files and embed them in the package together with
  the content of the /LICENSES folder. As this step can take a while
  it is only performed when building the final package in the publishing
  workflow.

• Adding a new workflow to ensure the project remains REUSE compliant.

[mknejp]

@TheBlubb14 @jvbsl as you are existing authors of the project please respond with a comment whether you approve of these licensing changes or if you would like anything changed.

[jvbsl]

All good. Approved from me

[TheBlubb14]

LGTM