Add configurable `icon_separator` to control spacing between icon and entry text

[chetan-satpute]

Is your feature request related to a problem? Please describe.
With the font I'm using, icons appear too close to the entry text in Telescope. The default separator doesn't provide enough spacing, hurting the visual alignment.

Font: Ubuntu Mono
Icons Font: Symbols Nerd Font Mono
Terminal: Kitty

Describe the solution you'd like
I explored the Telescope codebase and found that the spacing between the icon and entry text is a single space by default. To make this configurable, I introduced a new option called icon_separator, which allows users to define their own separator between the icon and the entry text.

Describe alternatives you've considered
I didn’t want to change my text font just to work around the spacing issue. Introducing a configurable icon_separator felt like a cleaner and more flexible solution, so I’m planning to keep using it.

Additional context
I've opened a pull request with this change. If the community finds it useful to configure the spacing between the icon and entry text, this option might help.

Attaching screenshots showing the improved spacing and use of custom separators.

config:

pickers = {
    find_files = {
        icon_separator = " ::",
    },
    live_grep = {
        icon_separator = " ::",
    },
    buffers = {
        icon_separator = " ::"
    }
}