Added support for .promptignore and custom ignore files

[lins05]

This PR implements .promptignore file support, allowing users to exclude files from LLM prompts without affecting version control.

Also supports arbitrary ignore files through new flag -E,--extra-ignore-files

Typical use case: to exclude assets (images etc. and generated code) from the output.

• These files need to be tracked by git, but no need to send to LLM

Why the the existing support for .ignore is not enough?

• It serves different purpose: If we put files in .ignore, it would make them ignored by searchers like silver searcher and rg
• No global ~/.ignore support

Implementation

This pull request adds support for .promptignore and custom ignore files to provide more flexible file filtering.

Key features:

• .promptignore Support: Automatically respects local and global .promptignore files.
• Custom Ignore Files: Use other ignore files like .dockerignore via the --extra-ignore-files flag.
• Disable .promptignore: A --no-promptignore flag has been added to bypass this feature.