feat: `strftime` for datetime namespace

[FBruzzesi]

What type of PR is this? (check all applicable)

• [ ] 💾 Refactor
• [x] ✨ Feature
• [ ] 🐛 Bug Fix
• [ ] 🔧 Optimization
• [ ] 📝 Documentation
• [ ] ✅ Test
• [ ] 🐳 Other

Checklist

• [x] Code follows style guide (ruff)
• [x] Tests added
• [x] Documented the changes

If you have comments or can explain your changes, please do so below.

• # noqa: A002 is because ruff complain about the "format" naming, but that's what polars uses.
• other comments in the code itself

[FBruzzesi]

Regarding CI failing:

-  "2020/03/01 00:00:00"
-  "2020/04/01 00:00:00"
-  "2020/05/01 00:00:00"
+  "2020/03/01 00:…
+  "2020/04/01 00:…
+  "2020/05/01 00:…

Not sure if... should I just trim the values?!

[MarcoGorelli]

In Polars, there's .dt.to_string, which does the same thing. Polars also has .dt.strftime as an alias...it's one of the very few aliases in Polars.

I think here (Narwhals), because we're stricter, we should probably just pick one

For the reverse operation, we have .str.to_datetime, not .str.strptime. So, I think for this one, we should probably use .dt.to_string?

[FBruzzesi]

Fair enough! Let me rename those 😁

[FBruzzesi]

I felt left out without a "ship it" gif 😂

[MarcoGorelli]

😄 not sure why it's not showing