feat(userreg): Add user-defined registers to Neovim.

This change unlocks additional registers for Neovim by allowing a user
to define their own behavior for non-builtin registers.

This is accopmlished through a new option 'userregfunc'

The 'userregfunc' defines the function to call when handling a register
for which there is no builtin functionality.

The 'userregfunc' function should take 3 arguments:

   action - Either "yank" or "put"
   register - The character corresponding to the register
   content - In the case of action == "yank", the dictionary describing
             the yanked content, with the following keys:

             {type} - Either "char", "line" or "block"
             {lines} - The lines being yanked as a list
             {width} - The width in case of "block" mode.
             {additional_data} - Additional data (can be returned in
             "put" mode)

   In case of "put" this function should return the content to put. This
   content can be either:

     * A dictionary in the same template as content above.
     * A list of strings. This will be assumed to be "line" mode.
     * A string. This will be assumed to be "char" mode.

An example of a "null" 'userregfunc' that provides an implementation
identical to traditional vim registers would be:

      let s:contents = {}
      function! MyUserregFunction(action, register, content) abort
         if a:action == "put"
           return get(s:contents, a:register, "")
        else
           let s:contents[a:register] = a:content
        endif
      endfunction
      set userregfun=MyUserregFunction

  It is important to note that any valid unicode character can now be a
  register, including something like @☺.

  This change also addresses the multibyte parsing issues surrounding

  let @a = 'xyz'
  let @🔨 = 'hammer'

1 files changed, 4 insertions, 6 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index cb46e26f82..05497e86ce 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -3061,12 +3061,10 @@ static int eval7(char **arg, typval_T *rettv, int evaluate, int want_string)
   // Register contents: @r.
   case '@':
     (*arg)++;
+    int regname = mb_cptr2char_adv((const char_u**) 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;
 
@@ -4166,7 +4164,7 @@ bool garbage_collect(bool testing)
 
   // registers (ShaDa additional data)
   {
-    const void *reg_iter = NULL;
+    iter_register_T reg_iter = ITER_REGISTER_NULL;
     do {
       yankreg_T reg;
       char name = NUL;
@@ -4175,7 +4173,7 @@ bool garbage_collect(bool testing)
       if (name != NUL) {
         ABORTING(set_ref_dict)(reg.additional_data, copyID);
       }
-    } while (reg_iter != NULL);
+    } while (reg_iter != ITER_REGISTER_NULL);
   }
 
   // global marks (ShaDa additional data)