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/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',