feat(defaults): mouse=nvi #19290

Problem:
Since right-click can now show a popup menu, we can provide messaging to
guide users who expect 'mouse' to be disabled by default. So 'mouse' can
now be enabled by default.

Solution:
Do it.
Closes #15521

src/nvim/main.c

@@ -356,7 +356,14 @@ int main(int argc, char **argv)
     abort();  // unreachable
   }
 
-  init_default_mappings();  // Default mappings.
+  // Default mappings (incl. menus)
+  Error err = ERROR_INIT;
+  Object o = nlua_exec(STATIC_CSTR_AS_STRING("return vim._init_default_mappings()"),
+                       (Array)ARRAY_DICT_INIT, &err);
+  assert(!ERROR_SET(&err));
+  api_clear_error(&err);
+  assert(o.type == kObjectTypeNil);
+  api_free_object(o);
   TIME_MSG("init default mappings");
 
   init_default_autocmds();

src/nvim/mapping.c

@@ -2113,24 +2113,6 @@ void f_mapcheck(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   get_maparg(argvars, rettv, false);
 }
 
-void init_default_mappings(void)
-{
-  add_map("Y", "y$", MODE_NORMAL, false);
-
-  // Use normal! <C-L> to prevent inserting raw <C-L> when using i_<C-O>
-  // See https://github.com/neovim/neovim/issues/17473
-  add_map("<C-L>", "<Cmd>nohlsearch<Bar>diffupdate<Bar>normal! <C-L><CR>",
-          MODE_NORMAL, false);
-  add_map("<C-U>", "<C-G>u<C-U>", MODE_INSERT, false);
-  add_map("<C-W>", "<C-G>u<C-W>", MODE_INSERT, false);
-  add_map("*", "y/\\\\V<C-R>\"<CR>", MODE_VISUAL, false);
-  add_map("#", "y?\\\\V<C-R>\"<CR>", MODE_VISUAL, false);
-
-  // Use : instead of <Cmd> so that ranges are supported (e.g. 3& repeats the substitution on the
-  // next 3 lines)
-  add_map("&", ":&&<CR>", MODE_NORMAL, false);
-}
-
 /// Add a mapping. Unlike @ref do_map this copies the string arguments, so
 /// static or read-only strings can be used.
 ///

src/nvim/options.lua

@@ -1597,7 +1597,7 @@ return {
       short_desc=N_("the use of mouse clicks"),
       type='string', list='flags', scope={'global'},
       varname='p_mouse',
-      defaults={if_true=""}
+      defaults={if_true="nvi"}
     },
     {
       full_name='mousefocus', abbreviation='mousef',
@@ -1619,7 +1619,7 @@ return {
       short_desc=N_("changes meaning of mouse buttons"),
       type='string', scope={'global'},
       varname='p_mousem',
-      defaults={if_true="extend"}
+      defaults={if_true="popup_setpos"}
     },
     {
       full_name='mousescroll',