feat: add cursorlineopt config option for styling livegrep file preview

[bansheerubber]

This is my first time contributing not only Lua code but also to a Neovim plugin, please let me know if changes are needed.

Summary
The live_grep file preview only highlights the current line number in the file preview instead of the entire line. I was having a tough time picking out which line matched my grep query in the file preview. I figured other people might like this feature too, so I decided to implement this as a config option that others can use.

I added the cursorlineopt property to the config.preview table since I figure it makes the most sense there (I can move it somewhere else if desired). The config option follows Neovim's cursorlineopt rules which can be found here. If cursorlineopt is not set, or if an invalid option is provided, then cursorlineopt defaults to both (as opposed to defaulting to whatever the global value of cursorlineopt is set to, which seems to be what fff.nvim currently does)

Before

After (using cursorlineopt = both)

[Copilot]

Pull request overview

Adds a new config.preview.cursorlineopt option to control how the cursorline is highlighted in the live_grep preview window, improving match readability in grep previews.

Changes:

• Introduces preview.cursorlineopt configuration with default 'both'.
• Applies the configured cursorlineopt to the preview window when in grep mode.
• Documents the new option in README.md.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
lua/fff/picker_ui.lua	Sets cursorlineopt on the preview window in grep mode using resolved config value.
lua/fff/conf.lua	Adds preview.cursorlineopt to config schema + default config.
README.md	Documents the new preview.cursorlineopt option in the setup example.

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[dmtrKovalenko]

It has always highlighted the line for me and I can't see any difference with the code but it makes sense so I'll just merge it