improve unpack_list error

[sorhawell]

resolves #600

this PR improves the bad error message for trailing commas for internal function unpack_list().

Also the function now has .context= and .call args just as unwrap().

I have provided context for any use of unpack_list()

In future when/if extendr+polars supports/allows trailing commas. This extra error styling can be removed again.

examples

> unpack_list(1,2,)
Error: Execution halted with the following contexts
0: During function call [unpack_list(1, 2, )]
1: trailing argument commas are not (yet) supported with polars
2: argument is missing, with no default
> pl$DataFrame()$select(1,2,)
Error: Execution halted with the following contexts
0: In R: in $select()
0: During function call [pl$DataFrame()$select(1, 2, )]
1: trailing argument commas are not (yet) supported with polars
2: argument is missing, with no default

[etiennebacher]

LGTM, can you add some tests and bump news?

[eitsupi]

Thanks for working on this.

Could you also update unpack_bool_expr and related functions?

[sorhawell]

> pl$DataFrame(list(list(1,2,3)))
shape: (3, 1)
┌────────────┐
│ new_column │
│ ---        │
│ list[f64]  │
╞════════════╡
│ [1.0]      │
│ [2.0]      │
│ [3.0]      │
└────────────┘
> pl$DataFrame(list(list(1,2,3)),"a")
shape: (1, 2)
┌───────────────────────┬──────────────┐
│ new_column            ┆ new_column_1 │
│ ---                   ┆ ---          │
│ list[list[f64]]       ┆ str          │
╞═══════════════════════╪══════════════╡
│ [[1.0], [2.0], [3.0]] ┆ a            │
└───────────────────────┴──────────────┘

[etiennebacher]

Apart from moving the functions dealing with ... in a separate dotdotdot.R, is there anything else to do here?