feat(make_entry): add icon_separator option

[chetan-satpute]

Description

Adds a new icon_separator option to allow users to customize the spacing between the icon and entry text in pickers. This improves visual alignment and allows users to customize the entry appearance.

Fixes #3483

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Manually tested with find_files, live_grep, and buffers pickers using different icon_separator values.
• Confirmed default behavior is unchanged when icon_separator is not set.

Configuration:

• Neovim version (nvim --version):

$ nvim --version
NVIM v0.11.2
Build type: Release
LuaJIT 2.1.1748459687

• Operating system and version:

$ sw_vers
ProductName:		macOS
ProductVersion:		15.5
BuildVersion:		24F74

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)