Add an .editorconfig file

[oliverklee]

This helps contributors get the basic formatting options in their IDEs
automatically set right. (The setting for PHP files complies to the
PSR-12 standard.)

This change does not autoformat any files with these settings, though.

[oliverklee]

Hold on, I think Markdown should have lines no longer than 80 characters. I'll do some more research on best practices there.

[oliverklee]

I'm not quite sure about this. Currently, the CSS test files are indented with tabs, and so this setting would keep this tabs. Or we could (in a separate PR) convert the CSS files to also use spaces for consistency. I'd prefer the second option and would be okay with both. :-)

@westonruter @sabberworm What do you think?

[oliverklee]

And should we allow (semi-)unlimited line lengths for the CSS files (as some files have compressed CSS)?

[oliverklee]

I've looked through some more CSS files: Their indentation is are a colorful mixture of tabs, 2 spaces and 4 spaces. :-)

[sabberworm]

Well, adding an editorconfig doesn’t mean the files need to be reformatted, it’s just a hint to the IDE on how such a file should be formatted.
I think it’s OK for fixtures to differ in indentation since the parser needs to be able to handle all styles found in the wild. Some fixtures are just real-life cases of files that the parser couldn’t handle correctly, which is why they’re committed as-is.
Personally I vastly prefer tab indentation over spaces but I prefer consistency even more so I don’t think we need to special-case *.css here.

[oliverklee]

Okay, I've remove the CSS special case now and repushed.