feat(extras.rest): add keymap for changing environment (#5678)

## Description

Context:

- kulala.nvim is an HTTP client, which is used in extras.rest
- The kulala [default
keymaps](https://neovim.getkulala.net/docs/getting-started/default-keymaps)
include a keymap to "set environment" (eg to change from production to
staging)
- This keymap is missing in extras/rest.lua

In this PR:

This adds a keymap for `<leader>Re` in extras.rest.lua.

## Screenshots

This adds `e` here:

<img width="595" alt="image"
src="https://github.com/user-attachments/assets/f06f527c-731c-4f81-ad17-0018de9e95d0"
/>

## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
guidelines.

lua/lazyvim/plugins/extras/util/rest.lua

@@ -12,6 +12,7 @@ return {
       { "<leader>Rb", "<cmd>lua require('kulala').scratchpad()<cr>", desc = "Open scratchpad", ft = "http" },
       { "<leader>Rc", "<cmd>lua require('kulala').copy()<cr>", desc = "Copy as cURL", ft = "http" },
       { "<leader>RC", "<cmd>lua require('kulala').from_curl()<cr>", desc = "Paste from curl", ft = "http" },
+      { "<leader>Re", "<cmd>lua require('kulala').set_selected_env()<cr>", desc = "Set environment", ft = "http" },
       {
         "<leader>Rg",
         "<cmd>lua require('kulala').download_graphql_schema()<cr>",