feat: allow passing a slice to and expression with the [] indexing

[timsaucer]

Which issue does this PR close?

This is a user request but no issue was opened.

Rationale for this change

With this you can do slicing of arrays via expression. For example, suppose you have this data frame:

+----------------------+
| a                    |
+----------------------+
| [1.0, 2.0, 3.0, 3.0] |
| [4.0, 5.0, 3.0]      |
| [6.0]                |
+----------------------+

You could do df.with_column("a[1:3]", column("a")[1:3]) to get

+----------------------+------------+
| a                    | a[1:3]     |
+----------------------+------------+
| [1.0, 2.0, 3.0, 3.0] | [2.0, 3.0] |
| [4.0, 5.0, 3.0]      | [5.0, 3.0] |
| [6.0]                | []         |
+----------------------+------------+

What changes are included in this PR?

Updates __getitem__ to take slice.
Adds unit tests.

Are there any user-facing changes?

No

TODO

• Add to the online documentation
• Update function docstring

[alamb]

Thanks @timsaucer -- I have a suggestion about testing, but otherwise looks good to me

[alamb]

should we also test non literal expressions? Even if they are supposed to error?

[timsaucer]

They will not error. Unit test added to demonstrate.