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, 11 insertions, 2 deletions

diff --git a/src/nvim/ops.h b/src/nvim/ops.h
index 05893c9940..a456d68003 100644
--- a/src/nvim/ops.h
+++ b/src/nvim/ops.h
@@ -22,6 +22,7 @@ typedef int (*Indenter)(void);
 #define PUT_LINE_SPLIT   16     // split line for linewise register
 #define PUT_LINE_FORWARD 32     // put linewise register below Visual sel.
 #define PUT_BLOCK_INNER  64     // in block mode, do not add trailing spaces
+#define ITER_REGISTER_NULL 0
 
 /*
  * Registers:
@@ -37,7 +38,8 @@ typedef int (*Indenter)(void);
 // The following registers should not be saved in ShaDa file:
 #define STAR_REGISTER 37
 #define PLUS_REGISTER 38
-#define NUM_REGISTERS 39
+#define USER_REGISTERS_START 39
+#define NUM_REGISTERS USER_REGISTERS_START
 
 // Operator IDs; The order must correspond to opchars[] in ops.c!
 #define OP_NOP          0       // no pending operation
@@ -96,6 +98,8 @@ typedef enum {
   YREG_YANK,
   YREG_PUT,
 } yreg_mode_t;
+/// Returns a reference to a user-defined register.
+int get_userreg(const int regname);
 
 /// Convert register name into register index
 ///
@@ -118,10 +122,15 @@ static inline int op_reg_index(const int regname)
   } else if (regname == '+') {
     return PLUS_REGISTER;
   } else {
-    return -1;
+    return get_userreg(regname);
   }
 }
 
+struct yank_registers;
+typedef struct yank_registers yank_registers_T;
+
+typedef size_t iter_register_T;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ops.h.generated.h"
 #endif