feat: add name_on_tag, suggest_name, and confirm_untag prompts to toggle()

[awerebea]

Summary

Add three opt-in boolean settings that make toggle() interactive: prompt for a tag
name on creation (optionally pre-filled), and ask for confirmation before removal.

New settings:

• name_on_tag (boolean, default false): when true, toggle() shows a
  vim.ui.input prompt ("Tag name (optional):") before creating a tag. Empty
  input -> unnamed tag; <Esc> cancels without tagging.
• suggest_name (boolean, default false): when true (and name_on_tag is
  also true), the name prompt is pre-filled with the file stem of the current
  buffer. Regular files lose their extension (my_file.txt -> my_file);
  hidden files keep their full name (.config -> .config). The suggestion is
  editable. Has no effect when name_on_tag is false.
• confirm_untag (boolean, default false): when true, toggle() shows a
  vim.fn.confirm dialog before removing an existing tag. The default choice is
  No, so an accidental <CR> is safe.

Implementation notes:

• Fast path: when all three settings are false, the original single
  enter_with_save code path is taken with zero overhead.
• vim.ui.input is async (callback-based), so tag existence is checked first
  with enter_with_result, then the save is performed inside the appropriate
  sync or async branch.
• Only toggle() is affected. tag() and untag() are unchanged.

No breaking changes. All settings default to false; existing behavior is
preserved exactly.

-- Example
require("grapple").setup({
    name_on_tag   = true,  -- prompt for a name when tagging
    suggest_name  = true,  -- pre-fill with file stem (e.g. "my_file")
    confirm_untag = true,  -- confirm before untagging
})