add a .editorconfig file.

this will allow IDEs/editors to apply rules how files should be written.

basically its not needed, unless there are files that are not fixed by pre-commits as configured via https://github.com/pytest-dev/pytest-html/blob/master/.pre-commit-config.yaml

acceptance criteria:

• editorconfig rules do not no not contradict established rules of pre-commits
• to be continued, comments are welcome.