fix(grug-far): add support for GrugFarWithin so it gets loaded by default (#5772)

## Description

https://github.com/MagicDuck/grug-far.nvim is the default find and
replace plugin for LazyVim.

It recently added support for being able to do a find / replace within a
visual selection to limit the scope of a search.

It did that by introducing a new command called `:GrugFarWithin`. This
PR adds that as a `cmd` so it gets loaded by default. Without this PR,
you cannot call this command unless you first call `:GrugFar`. This was
identified by the author of Grug Far.

## Related Issue(s)

There is no issue in this repo, but this feature is related to:
https://github.com/MagicDuck/grug-far.nvim/issues/357

## Checklist

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

I also want to give a shout-out to @MagicDuck who is the author of the
plugin. He implemented this feature so fast and has been super helpful
in his repo's issue tracker.

lua/lazyvim/plugins/editor.lua

@@ -4,7 +4,7 @@ return {
   {
     "MagicDuck/grug-far.nvim",
     opts = { headerMaxWidth = 80 },
-    cmd = "GrugFar",
+    cmd = { "GrugFar", "GrugFarWithin" },
     keys = {
       {
         "<leader>sr",