Add license to all files

What is the new feature

Add license to all files.

How is it implemented

The license should comply with REUSE specification.
The licenses BSD-3-Clause, CC-BY-4.0, CC0-1.0, and MIT are used.

CC-BY-4.0 is used for document files:

• AUTHORS.txt
• README.md

CC0-1.0 is used for unimportant files:

• gitlab MR templates (bugfix.md, feature.md)
• .clang-format
• .gitignore

MIT is used for stacktrace in backtrace

The rest uses BSD-3-Clause

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!