Add .editorconfig and CI check for trailing whitespace

[DeinAlptraum]

The editorconfig should prevent trailing whitespaces and ensure a trailing newline in all files.
The CI adds another job that runs on PRs only, marking all spots in src (though ignoring src/lib, src/webSDK) changed in that revision that added trailing spaces. An example of the output in case of failure can be seen here.

This encompasses stage 1 & 2 from #873

Open question: Should the .editorconfig apply to all files, or should we limit this to the same folders as the CI check?

[barbeque-squared]

Open question: Should the .editorconfig apply to all files, or should we limit this to the same folders as the CI check?

Imo there's no point in spending time figuring out what would be the "perfect" editorconfig because if it turns out it needs changing, that's super easy. It's better than nothing at all and over time it'll just converge towards whatever "perfect" turns out to be.
Compare it to adding dozens of ultra exotic Lua functions: what we choose now will have effects years into the future. Editorconfig only applies until the next time it's changed.

[barbeque-squared]

Looks good!