clang-tidy config for short case statements

In https://github.com/google/s2geometry/pull/453#discussion_r2461308787 @smcallis suggests

```
AllowShortCaseLabelsOnASingleLine:    true

AlignConsecutiveShortCaseStatements:
  Enabled: true
  AcrossEmptyLines: true
  AcrossComments: true
  AlignCaseColons: false
```

We should see how consistently the existing code is formatted and if this is the rule in general.