nvim/key: add __newindex meta to bind.Group

This allows callers of the struct to perform the more normal '='
assignment and have sane behavior.

Previously, this would not do what one expects when used with bind.Bind
or other bind.Group(s): it merely adds them to the Group object
directly, not adding it to the ._children table or downward propagating
the Group's settings.

This is fixed here, by adding a __newindex metamethod to the bind.Group
object, which checks to see if the addition is a known Bind or Group
type, and does the right if it is, falling back to the previous behavior
if it isn't

nvim/lua/psoxizsh/key/bind.lua

@@ -63,6 +63,19 @@ setmetatable(Group, {
     return self.__children[name]
   end,
 
+  __newindex = function(self, name, new)
+    local t = type(new) == 'table' and new.__struct or nil
+
+    if t and t == Group.__struct or t == Bind.__struct then
+      -- We rely on public behavior here (__call meta method)
+      -- should perhaps refactor the logic out of that and directly
+      -- call it here
+      self { [name] = new }
+    else
+      rawset(self, name, new)
+    end
+  end,
+
   ---Group.__call meta method
   ---@param self BindGroup
   ---@param new table<string, Bind|BindGroup|BindOptions>