add support for pl$concat(<LazyFrame>, . . . ) + add to_supertypes auto casting

[sorhawell]

closes #386

extras:

allow syntax pl$concat(. . .) and not only pl$concat(list(. . .)), this is matches behavoir for pl$DataFrame and $select().
to facility unit testing also implemented <DataFrame>$n_chunks() to count chunks
add robj_to! DataFrame and LazyFrame
non-breaking upgrades to internal functions unpack_list() [skip unpack if these classes] + get_err_ctx() [pick only this context type] + Plain_err() [allow multiline txt input]

minor deviation from py-polars

This PR deviate slightly from py-polars pl.concat which is a quite verbose implementation:
how = "align" is not implemented (it could be but it is just another task.)
how = "xyz_releaxed" are just accessed directly via arg to_supertypes which will cast differing types to any shared supertype.
r-polars do support to_supertypes also for concat Series, py-polars has not implemented that. Maybe that is why they don't just expose to_supertypes.

to_supertypes = TRUE is like R where c(1, "b") yields [1] "1" "b" and to_supertypes = FALSE will throw errors if not exactly the same type.

[etiennebacher]

Thanks, the Rust side looks good but I have several comments on the R side because for now this doesn't match entirely the behavior of py-polars

[sorhawell]

@etiennebacher I think we have addressed all first round of comments.

[etiennebacher]

Thanks, I think this should be good but I'll just do some tweaks in docs and fix typos