Add support for project instruction file

[yamadashy]

This PR is adding support for project instruction file.

related: #40

Changes

• Add instructionFilePath option to configuration
  • Update output generators to include project instruction in the output
• Rename "user-provided header" to "project summary" for clarity

README
https://github.com/yamadashy/repopack/tree/feature/custom-instruction?tab=readme-ov-file#using-instructionfilepath-option

Purpose

This PR adds support for a separate project instruction file, allowing users to provide more detailed context and guidelines for AI analysis.

[yamadashy]

I've been testing various approaches, including explicit instructions like:

## Important Rules and Conventions
1. Add as many comments as possible

However, getting consistent adherence is tricky. I'm still exploring better methods to improve this.

[yamadashy]

I've decided to experimentally introduce this Custom Instruction feature.
The reason for this is that we've already tested it with several feature implementation proposals, and it seems to be working well in enhancing AI's understanding and suggestions for the project.