Add GitHub actions workflows for unit testing, checking formatting, and updating actions

[msimberg]

• Adds a workflow that builds and runs the unit test in debug and release mode. It enables warnings and -Werror for uenv itself.
• Adds a workflow that checks formatting with clang-format.
• Adds a dependabot workflow that checks for actions updates daily.

[msimberg]

@bcumming I don't know if I messed up the workflow configuration files between testing and opening the PR. They ran on my fork so you may need to enable something in the settings to allow builds from forks or similar? Or add me to the maintainers?