feat: add "default" field to maparg() return value

With so many default mappings, plugins are not able to detect if an
existing mapping is one created by the user or one created by Nvim.

Add a "default" field to the dict return value of maparg() that plugins
can use to determine if a mapping is a default mapping provided by Nvim
(which they can safely override).

src/nvim/lua/executor.c

@@ -46,6 +46,7 @@
 #include "nvim/lua/treesitter.h"
 #include "nvim/macros_defs.h"
 #include "nvim/main.h"
+#include "nvim/mapping.h"
 #include "nvim/memline.h"
 #include "nvim/memory.h"
 #include "nvim/memory_defs.h"
@@ -2358,11 +2359,17 @@ void nlua_init_defaults(void)
   lua_State *const L = global_lstate;
   assert(L);
 
+  // All maps created here are marked as default maps.
+  map_set_default(true);
+
   lua_getglobal(L, "require");
   lua_pushstring(L, "vim._defaults");
   if (nlua_pcall(L, 1, 0)) {
     fprintf(stderr, "%s\n", lua_tostring(L, -1));
   }
+
+  // All maps created after this function exits are not default maps.
+  map_set_default(false);
 }
 
 /// check lua function exist

src/nvim/mapping.c

@@ -132,6 +132,16 @@ static const char e_entries_missing_in_mapset_dict_argument[]
 static const char e_illegal_map_mode_string_str[]
   = N_("E1276: Illegal map mode string: '%s'");
 
+/// Whether new mappings should be marked as "default". This is modified only in map_set_default().
+static bool create_default_maps;
+
+/// Enable or disable creationg of default mappings.
+///
+/// @param value True if new mappings should be marked as default mappings.
+void map_set_default(bool value) {
+  create_default_maps = value;
+}
+
 /// Get the start of the hashed map list for "state" and first character "c".
 mapblock_T *get_maphash_list(int state, int c)
 {
@@ -541,6 +551,8 @@ static void map_add(buf_T *buf, mapblock_T **map_table, mapblock_T **abbr_table,
     mp->m_desc = xstrdup(args->desc);
   }
 
+  mp->m_default = create_default_maps;
+
   // add the new entry in front of the abbrlist or maphash[] list
   if (is_abbr) {
     mp->m_next = *abbr_table;
@@ -2118,6 +2130,7 @@ static Dictionary mapblock_fill_dict(const mapblock_T *const mp, const char *lhs
   PUT_C(dict, "mode", CSTR_AS_OBJ(mapmode));
   PUT_C(dict, "abbr", INTEGER_OBJ(abbr ? 1 : 0));
   PUT_C(dict, "mode_bits", INTEGER_OBJ(mp->m_mode));
+  PUT_C(dict, "default", BOOLEAN_OBJ(mp->m_default));
 
   return dict;
 }

src/nvim/mapping_defs.h

@@ -24,4 +24,5 @@ struct mapblock {
   sctx_T m_script_ctx;      ///< SCTX where map was defined
   char *m_desc;             ///< description of mapping
   bool m_replace_keycodes;  ///< replace keycodes in result of expression
+  bool m_default;           ///< true if this is a |default-mapping|
 };