feat(snippet): set snippet keymaps permanent instead of dynamic (#31887)

Problem:

Given that `vim.snippet.expand()` sets temporary `<tab>`/`<s-tab>`
keymaps there is no way to build "smart-tab" functionality where `<tab>`
chooses the next completion candidate if the popup menu is visible.

Solution:

Set the keymap permanent in `_defaults`.

The downside of this approach is that users of multiple snippet engine's
need to adapt their keymaps to handle all their engines that are in use.
For example:

    vim.keymap.set({ 'i', 's' }, "<Tab>", function()
      if foreign_snippet.active() then
        return "<Cmd>lua require('foreign_snippet').jump()<CR>"
      elseif vim.snippet.active({ direction = 1 }) then
        return "<Cmd>lua vim.snippet.jump(1)<CR>"
      else
        return key
      end
    end, { expr = true })

Upside is that using `vim.keymap.set` to override keymaps is a well
established pattern and `vim.snippet.expand` calls made by nvim itself
or plugins have working keymaps out of the box.


Co-authored-by: Maria José Solano <majosolano99@gmail.com>

test/functional/plugin/lsp/completion_spec.lua

@@ -1241,7 +1241,9 @@ describe('vim.lsp.completion: integration', function()
         }
       end)
     )
-    feed('<tab>')
+    exec_lua(function()
+      vim.snippet.jump(1)
+    end)
     eq(
       #'hello friends',
       exec_lua(function()