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:

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