feat(fzf-lua): preview keymaps for git (#3904)

## What is this PR for?
It seems that the preview keymaps set right now only support the builtin
previewer.

From searching through `fzf-lua` docs, I found
[here](3b91c1a471/README.md (L634-L638))
that additional mappings should be set for the other previewers such as
`git` (otherwise the defaults `<shift-down/up>` could be used).

I chose to use the same mappings for the `fzf` preview keymaps as they
serve the same functionality. Please feel free to change if not to your
liking.
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## Does this PR fix an existing issue?
Not an issue but a discussion #3895
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  If this PR fixes any issues, please link to the issue here.
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## Checklist

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

lua/lazyvim/plugins/extras/editor/fzf.lua

@@ -53,6 +53,8 @@ return {
       config.defaults.keymap.fzf["ctrl-u"] = "half-page-up"
       config.defaults.keymap.fzf["ctrl-d"] = "half-page-down"
       config.defaults.keymap.fzf["ctrl-x"] = "jump"
+      config.defaults.keymap.fzf["ctrl-f"] = "preview-page-down"
+      config.defaults.keymap.fzf["ctrl-b"] = "preview-page-up"
       config.defaults.keymap.builtin["<c-f>"] = "preview-page-down"
       config.defaults.keymap.builtin["<c-b>"] = "preview-page-up"