Zen slicing

[LeandroVandari]

Implemented the feature in issue #3289.

Now, the !zen command accepts arguments in the form start:stop, and will return the corresponding zen lines.
Additionally, since the indices can now no longer simply be passed as integers, the zen_rule_index argument to the zen command was removed, and now indices are parsed from a regex expression.

[vivekashok1221]

Thank you for opening this PR 🐍

[LeandroVandari]

This latest commit fixes the problems you mentioned, but i noticed it's impossible to include the last line when slicing (i.e. !zen 1:18 and !zen 1:-1 will only include up to the semi-last line, and !zen 1:19 will error out). I was considering some options to fix that, and I wanted to get your opinions:

1. Make slicing end inclusive -- i don't like this because it feels unpythonic and just incoherent with the language
2. Allowing for end_index == len(zen_lines) -- again, weird and unpythonic, but less bad than the previous option.
3. Add the possibility for an open range (i.e. 1:) which will include up to the last line. -- this feels like the best option to me.

Additionally, I was wondering: Do we ever want to support steps, now that we're going the slice way? That feature would probably never be used, but it's just consistent with slicing I feel like.

[LeandroVandari]

Maybe it's possible to write a test for this? It seems like a mostly-pure function

I'm not really used to writing tests, especially for discord bots, but I could take a look at the code base and draft a PR for it later on (should I open an issue about it?)

[vivekashok1221]

2. Allowing for end_index == len(zen_lines) -- again, weird and unpythonic, but less bad than the previous option.
3. Add the possibility for an open range (i.e. 1:) which will include up to the last line. -- this feels like the best option to me.

I like option 2 since it's consistent with how slicing in Python works. If we deviate from it, people might think it's a bug.
Maybe we can do option 2 AND option 3 (your call).

re: writing tests

draft a PR for it later on (should I open an issue about it?)

Sure (you don't need to open an issue IMO but you can if you want to)
Writing tests is optional btw, but highly appreciated.

[LeandroVandari]

i implemented both 2 and 3, and from my quick tests all seems functional now. I'll take a look into the repo tests to see how I'd implement that

[vivekashok1221]

Noticed one more thing while testing. Everything else looks good to me.

[vivekashok1221]

LGTM

[LeandroVandari]

I drafted some tests for the zen command. I tried to keep a balance between hard-coding / developing answers myself. I don't think it's ready yet, but I'd love some opinions on it :)

[vivekashok1221]

LGTM