Improve completions generator

[DannyBen]

This PR updates Bashly's completions generator to emit Completely's new pattern configuration format instead of the older flat completion map.

The lower-level pattern matching and mixed-source behavior is implemented in Completely. See:

• Add a new pattern config format completely#103

Bashly Changes

• Added Bashly::CompletionBuilder to build Completely pattern config from the Bashly command tree.
• Kept the existing public completion APIs:
  • completion_data
  • completion_script
  • completion_function
• Updated command completion data to use:
  • patterns
  • option groups
  • named token sources
• Converted Bashly completion source syntax to Completely's explicit pattern source syntax:
  • <file> -> +file
  • <directory> -> +directory
  • <user> -> +user
  • static values stay static
  • static values beginning with + are escaped as ++value
• Preserved command-level completions as the fallback source for positional args without their own allowed or completions.
• Preserved allowed values as static token candidates.
• Added support in the generated pattern config for:
  • command aliases
  • global flags cascading to descendants
  • repeatable flags
  • repeatable positional args
  • positional args with no completion source
  • deterministic token names when different sources share the same arg/flag name.

Tests

• Updated completions approvals for the new pattern-config output.
• Added direct fixture-driven coverage for Bashly::CompletionBuilder.
• Kept the existing command-level completion specs as integration coverage for Command#completion_data, completion_script, and completion_function.
• Added small missing coverage for Settings.argfile_var.