Problem:
- `vim.highlight` module does not follow `:help dev-name-common`, which
  documents the name for "highlight" as "hl".
- Shorter names are usually preferred.

Solution:
Rename `vim.highlight` to `vim.hl`.

This is not a breaking change until 2.0 (or maybe never).

This mostly means replacing `nvim_buf_call` and `nvim_win_call` with
`vim._with`.

ref https://github.com/neovim/neovim/pull/28432
ref https://github.com/neovim/neovim/issues/28469

- Use getregionpos().
- Use a single extmark for non-blockwise selection.

Problem:
The nvim_win_xx_ns function family introduced in ba0370b1d718d473d0ef51c35d88b98ba220082b
needs more bake-time. Currently it's narrowly defined for windows, but
other scopes ("buffer") and features are likely in the future.

Solution:
- Rename the API with double-underscore to mark it as EXPERIMENTAL.

TODO/FUTURE:
- Rename and change the signature to support more than just "window"
  scope, and for other flexibility.
- Open question: we could choose either:
  - "store scopes on namespaces", or
  - "store namespaces on scopes (w:/b:/…)"

Problem:
nvim_win_remove_ns does not follow `help dev-naming` API naming conventions.

Solution:
Rename it.

This avoids redraw when adding/removing an empty namespace for a window.

This also avoids marktree traversal when clearing a namespace that has
already been cleared, which is added as a benchmark.

- 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>