Added comma for index + step in slice

[elrandira]

This is my proposition to add comma in index such as in $..books[1,2] and step in slice such as in $..books[::-1] or $..books[0:3:2]

for the comma in index, the code is similar to handling of comma in fields. #28

For step in slice, all the code was already there, just needed to implement the rule... ¯_(ツ)_/¯

All tests passed + new ones

[michaelmior]

@elrandira Sorry for the very long delay. If you're still interested in having this merged and you're able to update the PR, let me know!

[elrandira]

sure, I'll try in the next days. I am going to need to setup the environment again for this.

[kurtmckee]

This will conflict with a documented JSONPath syntax for selecting multiple indexes simultaneously:

https://goessner.net/articles/JsonPath/index.html#e3

This PR should be closed to avoid introducing conflicting syntax.

[kurtmckee]

I think I misread this; my interpretation was that this was offering commas as an alternative to colons in slice syntax, but now that I've read the code that doesn't appear to be what's happening here.

I recommend merging #145 first, which heavily parametrizes the test suite. It will make it easier to integrate the tests introduced here.