v0.7 Approaching 1.0

Starting with this release, demicode clearly distinguishes between user errors and unexpected exceptions, even if it internally uses exceptions for both. For the former, it only prints the error message. For the latter, it also prints an exception trace and points to the issue tracker. Demicode's output of statistics with the --stats option has been significantly improved as well.

The test script has been modularized using Python's builtin unittest module. You can run tests with ./runtest.py or with Visual Studio Code, the latter thanks to the configuration in .vscode/settings.json. In preparation of the release of Unicode 15.1, the versions for code generation have been locked down. In particular, testing grapheme cluster breaks now is specific to Unicode 15.0, since 15.1 updates the algorithm.