fix: update pivot expr on to first param

[b-twice]

The pivot function now has on as the first param instead of index, as noted in the python 1.0 upgrade documentation:

columns has been renamed to on, and is now the first positional argument.

Added some test coverage from examples in docs to capture this. Existing test coverage was using the same column as index and on, resulting in this not being caught.

[Bidek56]

1. Don't you have to change the Rust fn pivot_expr( to match it?
2. Did you run 'yarn run lints:ts` on your TS code?
   Thx

[b-twice]

1. Don't you have to change the Rust fn pivot_expr( to match it?

   2. Did you run 'yarn run lints:ts` on your TS code?
      Thx

1. 🤦‍♂️That would be helpful... done
2. Yes passes lint + tests:

[owlas]

Thanks for fixing this, our team was so confused that index + on where reversed. Is this in the latest release?