add .editorconfig file

[Fatpandac]

It changes the following:

• add .editorconfig file

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[naseschwarz]

Thank you very much for your PR.

I tested this and it works fine in neovim and editorconfig-checker. The latter finds a lot of misformats from rustfmt, so I think it's a good idea to have an .editorconfig in order to avoid these in the first place!

(I'll follow up with the rustfmt project and the hard_tabs issue separately.)

[extrawurst]

Thanks for the contribution! I am going to merge this without a changelog because it’s not end-user facing but ideally we add changelogs for even dx changes 👍