feature: Implement to_string in expr_dt

[ym-pett]

linked to #35

daft plays nicely with Narwhals on this one!

please note: I had to manually choose the tests to remove in run_tests.py, as python update_run_tests.py added lots of tests back in that we'd already removed and didn't remove the relevant tests. I don't quite understand why, maybe it's because locally I'd branched off the wrong branch to start with in the PR?

Let me know if this should be investigated as part of this PR. I'm tempted to think it's some leftover mess on my local version that will sort itself out with the next pull from main.

I did to the usual:

git fetch upstream
git reset --hard upstream/main

[MarcoGorelli]

thanks!

you might have missed git submodule update --init --recutsive after git pull?

[ym-pett]

aahhh that was exactly it, thanks for reminding me! you've unblocked me on the next PR I believe