fix(lang/r): make keymaps in which-key menu available in visual mode (#4565)

## Description
`r.nvim` keymaps don't show group names in visual mode.
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## Related Issue(s)
No, rather discussion #4564
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## Screenshots

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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
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lua/lazyvim/plugins/extras/lang/r.lua

@@ -22,6 +22,7 @@ return {
           local wk = require("which-key")
           wk.add({
             buffer = true,
+            mode = { "n", "v" },
             { "<localleader>a", group = "all" },
             { "<localleader>b", group = "between marks" },
             { "<localleader>c", group = "chunks" },