🚀 Add pipx installation support via pyproject.toml

[mgcooper]

This PR adds support for installing and running gpt-repository-loader via pipx, enabling:

pipx install git+https://github.com/mpoon/gpt-repository-loader.git
gpt-repository-loader /path/to/repo

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🔧 Summary of Changes

• ✅ Moved gpt_repository_loader.py into a new gpt_repository_loader/ package directory (required for pipx to correctly install)
• ✅ Moved the scripting inside the if name == main guard into a main() function and moved the guard to the end of gpt_repository_loader.py (this enables the gpt-repository-loader CLI script)
• ✅ Added a pyproject.toml with [project.scripts] entry for gpt-repository-loader = "gpt_repository_loader.gpt_repository_loader:main"
• ✅ Added [tool.setuptools] to restrict packaging to gpt_repository_loader, otherwise pipx interprets the test_data folder as a package
• ✅ Updated the import statement in the test script to reflect the new package layout (i.e., import from gpt_repository_loader.gpt_repository_loader instead of just gpt_repository_loader)

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🧪 Test Instructions

From the root of the project, you can now run:

pipx install --force --editable .
gpt-repository-loader /path/to/any/git/repo

To test directly from GitHub:

pipx install git+https://github.com/mgcooper/gpt-repository-loader.git@feat/pyproject.toml

If this PR is accepted, you can then install and access the cli tool:

pipx install git+https://github.com/mpoon/gpt-repository-loader.git

Several other PRs currently address the lack of a setup.py or pyproject.toml file. To my knowledge this is the most minimal refactor possible to address this, and enable pipx install, which many users will prefer.

Closes #63