Ref #21393

- Move default user commands to _defaults.lua as that now contains all
  kinds of defaults rather than just default mappings and menus.
- Remove the :aunmenu as there are no menus when _defaults.lua is run.

"Edit" more closely describes the generic application than "Preview", though
the buffer contents don't (yet) map to an actual file on disk.

https://github.com/neovim/neovim/pull/24703#discussion_r1321719133

Problem:
netrw may conflict with the Nvim default "gx" mapping.

Solution:
Initialize keymapping earlier by moving it to vim._init_default_mappings().
That also avoids needing to check maparg().

This matches netrw's use of maparg().

Problem:
Showing an error via vim.notify() makes it awkward for callers such as
lsp/handlers.lua to avoid showing redundant errors.

Solution:
Return the message instead of showing it. Let the caller decide whether
and when to show the message.

---
Rejected experiment: move vim.ui.open() to vim.env.open()

Problem:
`vim.ui` is where user-interface "providers" live, which can be
overridden. It would also be useful to have a "providers" namespace for
platform-specific features such as "open", clipboard, python, and the other
providers listed in `:help providers`. We could overload `vim.ui` to
serve that purpose as the single "providers" namespace, but
`vim.ui.nodejs()` for example seems awkward.

Solution:
`vim.env` currently has too narrow of a purpose. Overload it to also be
a namespace for `vim.env.open`.

diff --git a/runtime/lua/vim/_meta.lua b/runtime/lua/vim/_meta.lua
index 913f1fe20348..17d05ff37595 100644
--- a/runtime/lua/vim/_meta.lua
+++ b/runtime/lua/vim/_meta.lua
@@ -37,8 +37,28 @@ local options_info = setmetatable({}, {
   end,
 })

-vim.env = setmetatable({}, {
-  __index = function(_, k)
+vim.env = setmetatable({
+  open = setmetatable({}, {
+      __call = function(_, uri)
+        print('xxxxx'..uri)
+        return true
+      end,
+      __tostring = function()
+        local v = vim.fn.getenv('open')
+        if v == vim.NIL then
+          return nil
+        end
+        return v
+      end,
+    })
+  },
+  {
+  __index = function(t, k, ...)
+    if k == 'open' then
+      error()
+      -- vim.print({...})
+      -- return rawget(t, k)
+    end
     local v = vim.fn.getenv(k)
     if v == vim.NIL then
       return nil

Co-authored-by: Mathias Fußenegger <mfussenegger@users.noreply.github.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>
Co-authored-by: ii14 <59243201+ii14@users.noreply.github.com>

Problem:
vim.treesitter.inspect_tree() and :InspectTree does not respect 'splitright'.

Solution:
- Change the default `command` from `topleft 60vnew` to `60vnew`.
- Change :InspectTree to respect command mods (`:vertical`, count, etc.).

Closes #22656