feat(picker): Add preview window support for telescope/fzf-lua/snacks

[mhiro2]

Summary

This PR adds file preview window support to external picker backends (telescope, fzf-lua, and snacks.nvim) when selecting locations in peekstack.

Changes

• Added shared external picker item metadata in util/picker:
  • file
  • lnum (1-based)
  • col (1-based for telescope/fzf-lua source data)
• Enabled Telescope file preview by passing filename/position metadata and using a previewer.
• Updated fzf-lua entries to include file/line/column data while keeping stable selection behavior for
  duplicate labels.
• Updated snacks picker items to use file-based format for preview and fixed key collision:
  • replaced custom loc payload with peekstack_loc
  • used pos = { line, col } in snacks-compatible shape
• Added/updated tests for telescope, fzf-lua, and snacks picker behavior.
• Updated README and help docs to describe external picker preview behavior.