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'

2 files changed, 10 insertions, 5 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 9c3c859771..1e39042a53 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -3673,6 +3673,7 @@ static void f_has(typval_T *argvars, typval_T *rettv, FunPtr fptr)
     "winaltkeys",
     "writebackup",
     "nvim",
+    "userreg",
   };
 
   // XXX: eval_has_provider() may shell out :(
diff --git a/src/nvim/eval/vars.c b/src/nvim/eval/vars.c
index b38849730a..73e84a8c1e 100644
--- a/src/nvim/eval/vars.c
+++ b/src/nvim/eval/vars.c
@@ -388,7 +388,7 @@ const char *skip_var_list(const char *arg, int *var_count, int *semicolon)
 static const char *skip_var_one(const char *arg)
 {
   if (*arg == '@' && arg[1] != NUL) {
-    return arg + 2;
+    return arg + 1 + utfc_ptr2len(arg + 1);
   }
   return (char *)find_name_end(*arg == '$' || *arg == '&' ? arg + 1 : arg,
                                NULL, NULL, FNE_INCL_BR | FNE_CHECK_START);
@@ -692,10 +692,14 @@ static char *ex_let_one(char *arg, typval_T *const tv, const bool copy, const bo
       return NULL;
     }
     arg++;
+
+    int regname = utf_ptr2char(arg);
+    int mblen = utf_ptr2len(arg);
+
     if (op != NULL && vim_strchr("+-*/%", *op) != NULL) {
       semsg(_(e_letwrong), op);
     } else if (endchars != NULL
-               && vim_strchr(endchars, *skipwhite(arg + 1)) == NULL) {
+               && vim_strchr(endchars, *skipwhite(arg + mblen)) == NULL) {
       emsg(_(e_letunexp));
     } else {
       char *s;
@@ -703,7 +707,7 @@ static char *ex_let_one(char *arg, typval_T *const tv, const bool copy, const bo
       char *ptofree = NULL;
       const char *p = tv_get_string_chk(tv);
       if (p != NULL && op != NULL && *op == '.') {
-        s = get_reg_contents(*arg == '@' ? '"' : *arg, kGRegExprSrc);
+        s = get_reg_contents(*arg == '@' ? '"' : regname, kGRegExprSrc);
         if (s != NULL) {
           ptofree = (char *)concat_str((char_u *)s, (const char_u *)p);
           p = (const char *)ptofree;
@@ -711,9 +715,9 @@ static char *ex_let_one(char *arg, typval_T *const tv, const bool copy, const bo
         }
       }
       if (p != NULL) {
-        write_reg_contents(*arg == '@' ? '"' : *arg,
+        write_reg_contents(*arg == '@' ? '"' : regname,
                            (const char_u *)p, (ssize_t)STRLEN(p), false);
-        arg_end = arg + 1;
+        arg_end = arg + mblen;
       }
       xfree(ptofree);
     }