feat(userregfunc): programmable user-defined registers with multibyte support

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.

1 files changed, 6 insertions, 7 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 3f73074333..370565c441 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -3446,12 +3446,10 @@ static int eval7(char **arg, typval_T *rettv, evalarg_T *const evalarg, bool wan
   // Register contents: @r.
   case '@':
     (*arg)++;
+    int regname = mb_cptr2char_adv((const char**) arg);
     if (evaluate) {
       rettv->v_type = VAR_STRING;
-      rettv->vval.v_string = get_reg_contents(**arg, kGRegExprSrc);
-    }
-    if (**arg != NUL) {
-      (*arg)++;
+      rettv->vval.v_string = get_reg_contents(regname, kGRegExprSrc);
     }
     break;
 
@@ -7073,16 +7071,17 @@ void set_argv_var(char **argv, int argc)
 /// Set v:register if needed.
 void set_reg_var(int c)
 {
-  char regname;
+  int regname;
 
   if (c == 0 || c == ' ') {
     regname = '"';
   } else {
-    regname = (char)c;
+    regname = c;
   }
+
   // Avoid free/alloc when the value is already right.
   if (vimvars[VV_REG].vv_str == NULL || vimvars[VV_REG].vv_str[0] != c) {
-    set_vim_var_string(VV_REG, &regname, 1);
+    set_vim_var_string(VV_REG, reg_to_mb(regname), -1);
   }
 }