Add .gitignore

There should be a .gitignore file for common files and dirs cf. https://github.com/fmtlib/fmt/blob/master/.gitignore

We could start with a minimal ignore list and go from there:

.cache/
.vscode/
build/
compile_commands.json

Regards, Martin

Martin Stump martin.stump@mercedes-benz.com on behalf of Mercedes-Benz Tech Innovation GmbH, Provider Information