Normalize whitespace

[DavyLandman]

This PR:

• normalizes whitespaces (30% of the lines were tabs, now it's all 4 spaces)
• removed trailing whitespace
• normalize indentation (it was mostly 4 spaces, but some parts were 2 spaces or tabs/space mixed)
• add a check around common java idioms
• verify at PR time nothing is introduced that we don't want (but be very conservative of the rules we user)

[github-actions]

Test Results

     99 files  ±0       99 suites  ±0   5m 48s ⏱️ +6s
242 302 tests ±0  242 301 ✅ ±0  1 💤 ±0  0 ❌ ±0 
726 993 runs  ±0  726 990 ✅ ±0  3 💤 ±0  0 ❌ ±0 

Results for commit decd011. ± Comparison against base commit b4a6545.

[jurgenvinju]

This is great. Very annoying that these things keep creeping in. I blame the constant hidden evolution of editor settings. Now that's at least stopped. And fixed. Thanks!