Refuse trailing whitespace in build

[odersky]

I see increasing occurrences of trailing whitespaces in the dotty code base. This should be avoided since it messes up histories and diffs.

1. Everyone: Please configure your editor so that trailing whitespace are stripped on save
2. We should have a PR check that refuses to merge a PR if there is trailing whitespace

We used to have that at some point, but it seems it's no longer active.

[soronpo]

I think the dotty repo should have a .scalafmt config and enforce it as a whole. I imagine it may be a drastic diff since the codebase is currently very inconsistent.