feat: add support for 'file_encoding' option

[staticssleever668]

Following telescope.builtin.live_grep().

NOTE: grep_previewer already uses this option.

Description

Implements 'file_encoding' option like in telescope.builtin.live_grep().
Without this previewer converts and breaks encoding, so this is also a bug fix.
The option is documented in telescope.builtin.live_grep(), so I didn't touch README.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

echo КОТ > utf-8
echo КОТ | iconv -t cp866 > cp866

telescope.extensions.live_grep_args.live_grep_args({file_encoding = "cp866"}) typing КОТ finds 'cp866'.
telescope.extensions.live_grep_args.live_grep_args({}) typing КОТ finds 'utf-8'.

Configuration:

• Neovim version (nvim --version): v0.10.1
• Operating system and version: NixOS 24.05 + unstable

Checklist:

• My code follows the style guidelines of this project (stylua)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (lua annotations)