Fix 'case' with newlines before expression

[aibaars]

This PR fixes syntax errors in case statements with a newline before its value expression. For example

case
   foo
when ....
end

Checklist:

• All tests pass in CI.
• There are sufficient tests for the new fix/feature.
• Grammar rules have not been renamed unless absolutely necessary.
• The conflicts section hasn't grown too much.
• The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[hendrikvanantwerpen]

LGTM!