Update the pythonic example to demonstrate how to create a flatten token stream from a token tree

[tatsuya6502]

This PR adds a step to the pythonic example to convert a token tree from the lexer into a flat token stream using Stream::from_nested.

This would not be a long-term solution because it's quite verbose, but it will work fine for now.

Relates to #20.

[zesterer]

Thanks! I wonder whether the utility functions could be placed inline at their use site give that they're not particularly complex? I think it makes it easier to see what's going on (i.e: avoids confusion about them being functions from chumsky)

[tatsuya6502]

Thank you for reviewing the code! I will update my code in this weekend.

[zesterer]

@tatsuya6502 Are you happy for me to take this branch and make the changes myself?

[zesterer]

I've used the work in this PR as a reference when improving the example in 91e5cc8. Thanks!

[tatsuya6502]

@zesterer Thank you for updating the code and merging it! It is silly; I have been distrusted by other stuff recently. So I am very happy to see the code was merged. Thank you!

[zesterer]

Thanks so much for the initial work you did, it was very useful as a reference!