Modulize pyproject dependencies for mplang frontend and runtime

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Currently, mplang consists of several core components:

1. mplang frontend: Responsible for compiling source code into mpir format, should have minimal and independent dependencies.
2. mplang runtime: A reference implementation for running mpir, also requires minimal independent dependencies.
3. Development and example dependencies: For development and usage examples, separate from core components.

Proposal:

• Refactor the pyproject file to modularize dependencies for frontend and runtime, ensuring each part has its own explicit dependency section.
• Clarify and document the separation in the repository, increasing maintainability and transparency.
• Clearly distinguish between core, dev, and example dependencies in pyproject.

This will help contributors and users to better understand and maintain each part of mplang.