Rewriter of old to new settings format.

[wadoon]

This PR brings a tool for rewriting the settings in KeY files to the new JSON-like format. A requirement to get rid of the old property-based interface.

Changes along the road:

• The KeYLexer can be configured whether \proof is also treated as EOF or not. In normal mode, the lexer stops at \proof so that the s-expr of the proof are not going through the parser.

Plan

• Rewrite all KeY files. This happens in separate PR Rewriting the settings in KeY files to the new format. #3688

Type of pull request

• Refactoring (behaviour should not change or only minimally change)
  • Lexer became the possibility to not stop at \proof.

Ensuring quality

• Before overwriting the file, the reformatter checks the parseability.
  The rewriting itself is done via the settings classes (load/store methods), as we are not morons, this should ensure settings equality.

[mattulbrich]

Agreed during meeting: Move to scripts directory. Reminder: When removing from the parser, make sure to give a descriptive error message.