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, 2 insertions, 0 deletions

diff --git a/src/nvim/map.c b/src/nvim/map.c
index d3058a5d52..86e7317b56 100644
--- a/src/nvim/map.c
+++ b/src/nvim/map.c
@@ -163,6 +163,8 @@ static inline bool ColorKey_eq(ColorKey ae1, ColorKey ae2)
   return memcmp(&ae1, &ae2, sizeof(ae1)) == 0;
 }
 
+MAP_IMPL(ptr_t, int, DEFAULT_INITIALIZER)
+MAP_IMPL(int, ptr_t, DEFAULT_INITIALIZER)
 MAP_IMPL(int, int, DEFAULT_INITIALIZER)
 MAP_IMPL(int, cstr_t, DEFAULT_INITIALIZER)
 MAP_IMPL(cstr_t, ptr_t, DEFAULT_INITIALIZER)