Fix scanning of division vs regex before line ending

[aibaars]

This should improve parsing of expressions with a division (/) operator before a line break. Previously the scanner treated those as the start of a regular expression literal (/..../).

For example:

foo /
5

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).