fix(pack): rename confirmation buffer to again use nvim-pack:// scheme

Problem: `nvim://` scheme feels more like a generalized interface that
  may be requested externally, and it acts like CLI args (roughly).
  This is how `vscode://` works.

  Anything that behaves like an "app" or a "protocol" deserves its own
  scheme. For such Nvim-owned things they will be called `nvim-xx://`.

Solution: Use `nvim-pack://confirm#<bufnr>` template for confirmation
  buffer name instead of `nvim://pack-confirm#<bufnr>`.

runtime/doc/lua-plugin.txt

@@ -283,7 +283,7 @@ mappings to a specific action by invoking `vim.lsp.buf.code_action()` with the
 
 Example: See `runtime/lua/vim/pack/_lsp.lua` for how vim.pack defines an
 in-process LSP server to provide interactive features in its
-`nvim://pack-confirm` buffer.
+`nvim-pack://confirm` buffer.
 
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 Troubleshooting                                   *lua-plugin-troubleshooting*