fix: support negative index, multislice in tidyselect

[machow]

Fix tidyselect to support:

from siuba.data import mtcars
from siuba import _, select

# negative indexing
select(mtcars, -1)

# integers in extended slice syntax
select(mtcars, _[0, 1])

Additionally, negating in any position used to be treated as triggering an implicit everything(). Now, only a negation in the first position does that (consistent with R tidyselect)