refactor(vim.iter)!: rename xxback() => rxx()

[justinmk]

Open questions

• instead of renaming xxback => rxx, could remove xxback and let the various functions accept negative indexes. The tradeoff is that it's a bit less obvious that the negative indexes are only supported for list-iterators. With rxx(), it's easy to notice that "the rxx functions only accept list-iterators".

Problem:

vim.iter has both rfind() and various *back() methods, which work in "reverse" or "backwards" order. It's inconsistent to have both kinds of names, and "back" is fairly uncommon (rust) compared to python (rfind, rstrip, rsplit, …). #27953

Solution:

• Remove nthback() and let nth() take a negative index.
  • Because rnth() looks pretty obscure, and because it's intuitive for a function named nth() to take negative indexes.
• Rename xxback() methods to rxx().
  • Bonus: this groups the "list-iterator" functions under a common r prefix.
• Rename peekback() to pop().
  • pop is chosen in duality with the existing peek.

Alternatives

We could remove the xxback() functions and instead let the xx() variants accept negative indexes. That has some tradeoffs:

• complicates documentation a bit, because the "reverse" variants are only supported for list-like iterator (:help list-iterator)
• the "r" prefix has the small benefit of grouping the "family" of "reverse" functions.

closes #27953