Fix the issue where default input files may include non-existing files

[sile]

Closes #97

Copilot Generated Summary

This pull request primarily focuses on simplifying the file collection process in the src/files.rs file and adding a new dependency in the Cargo.toml file. The most significant changes include the addition of the ignore crate to the project dependencies, the use of the Walk struct from the ignore crate to collect files, and the removal of several functions related to file collection that are no longer necessary.

Dependency addition:

• Cargo.toml: Added ignore crate version "0.4.22" to the project dependencies. This crate provides the Walk struct used for file collection.

File collection simplification:

• src/files.rs: Imported the Walk struct from the ignore crate.
• src/files.rs: Simplified the collect_default_target_files function by using the Walk struct to collect files instead of the previous method, which involved checking if the current directory is a git repository and collecting files differently based on that. Removed the collect_files_with_git and collect_files_without_git functions as they are no longer necessary.
• src/files.rs: Removed the is_git_repository function as it is no longer needed due to the changes in the file collection process.