REPL: Moving left and right was skipping newlines

Issue #7210 fixed movement across combining characters, such that moving left
and right by one character would skip all zero-width characters, but this also
inadvertently includes newlines. This made it impossible to navigate to the end
of a line that ended with a newline with the left and right arrows alone. And movement across multiple newlines would skip across them all at once.

This fixes that and adds simple regression tests.
Merge pull request #7253 from mbauman/char-move-skips-newlines

REPL: Moving left and right was skipping newlines