This patch introduces a new global option `userregfunc`, allowing users
to define custom behavior for registers not handled by Neovim
internally. This enables programmable registers using any Unicode
character — including multibyte characters.

- A new register slot `USER_REGISTER` is introduced. Any register not
  matching the standard set (`0-9a-zA-Z"+-*%#/:.=`, etc.) is routed
  through this system.

- When such a register is accessed, the function defined in
  `userregfunc` is called with three arguments:

    1. `{action}` (string): either `"yank"` or `"put"`
    2. `{register}` (string): UTF-8 character name of the register
    3. `{content}`:
       - If `action == "yank"`: a dictionary with these keys:
         - `lines` (list of strings): the yanked text
         - `type` (string): one of `"v"` (charwise), `"V"` (linewise), or `"b"` (blockwise)
         - `width` (number, optional): present if `type == "b"`
         - `additional_data` (dict, optional): user-extensible metadata
       - If `action == "put"`: this is always `v:null`

- The function may return either:
    - A **string** (used as a charwise register), or
    - A **dictionary** matching the structure above

- Internally, `read_userregister()` and `write_userregister()` convert
  between `yankreg_T` and typval dictionaries.

- Messages and internal logic fully support multibyte register names via
  UTF-8.

- A new `USER_REGISTER` slot is used for logical separation in the
  register table.

Included in this patch is an extensible Lua framework (`vim.userregs`)
for defining user register handlers in Lua. It provides per-register
handlers via `register_handler(registers, handler)`

The global function `_G.def_userreg_func` is registered as the default
implementation of `'userregfunc'`, enabling seamless integration with
the Lua framework.

- Register `λ` dynamically inserts the current date
- Register `&` reads and writes from a "global register" file under
  `stdpath("run")`
- Register `?` returns the result of a shell command
- Registers that auto-adjust based on filetype, cursor context, or
  Treesitter nodes

This change expands the register model into a programmable abstraction —
fully scriptable and extensible — without breaking compatibility.

fix #33075

Problem:
- `health#check()` seems to have been removed for a while, but `:h faq`
  still refers to it.
- `news-0.11.txt` doesn't mention #33044

Problem:  cannot :put while keeping indent (Peter Aronoff)
Solution: add the :iput ex command (64-bitman)

fixes: vim/vim#16225
closes: vim/vim#16886

https://github.com/vim/vim/commit/e08f10a55c3f15b0b4af631908551d88ec4fe502

Cherry-pick test_put.vim changes from patch 8.2.1593.

N/A patches:
vim-patch:9.1.1213: cannot :put while keeping indent
vim-patch:9.1.1215: Patch 9.1.1213 has some issues

Co-authored-by: 64-bitman <60551350+64-bitman@users.noreply.github.com>
Co-authored-by: Hirohito Higashi <h.east.727@gmail.com>

Problem: Reconfiguring a float window applies the global 'winborder'.
Solution:
- Ignore 'winborder' when reconfiguring a float window.
- Still apply 'winborder' when converting a split to a float window.

While at it, rename the p_winbd variable to p_winborder, as 'winbd' is
not the option's short name.

Co-authored-by: glepnir <glephunter@gmail.com>

Problem:
There is currently no global option to define the default border style for floating windows. This leads to repetitive code when developers need consistent styling across multiple floating windows.

Solution:
Introduce a global option winborder to specify the default border style for floating windows. When a floating window is created without explicitly specifying a border style, the value of the winborder option will be used. This simplifies configuration and ensures consistency in floating window appearance.

Co-authored-by: Gregory Anders <greg@gpanders.com>