build(types): allow vararg returns in function types

The optimizations that vim.iter uses for array-like tables don't require
that the source table has no holes. The only thing that needs to change
is the determination if a table is "list-like": rather than requiring
consecutive, integer keys, we can simply test for (positive) integer
keys only, and remove any holes in the original array when we make a
copy for the iterator.

Closes https://github.com/neovim/neovim/issues/28484.
Closes https://github.com/neovim/neovim/issues/28719.

Co-authored-by: Chris <crwebb85@gmail.com>
Co-authored-by: Gregory Anders <greg@gpanders.com>
Co-authored-by: Jake B <16889000+jakethedev@users.noreply.github.com>
Co-authored-by: Jonathan Raines <jonathan.s.raines@gmail.com>
Co-authored-by: Yi Ming <ofseed@foxmail.com>
Co-authored-by: Zane Dufour <zane@znd4.me>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>

- Also delete old perl scripts which are not used since 8+ years ago.

fix #23251
fix #27367
ref https://github.com/neovim/neovim/issues/2252#issuecomment-1902662577

Helped-by: Daniel Kongsgaard <dakongsgaard@gmail.com>
Co-authored-by: Kevin Pham <keevan.pham@gmail.com>

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, …).

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()`.
  - This informally groups the "list-iterator" functions under a common
    `r` prefix, which helps discoverability.
- Rename `peekback()` to `pop()`, in duality with the existing `peek`.

Problem:
The use-case for the convenience functions vim.iter.map(),
vim.iter.filter(), vim.iter.totable() is not clear.

Solution:
Drop them for now. We can revisit after 0.10 release.

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