[IR][Buider] (2/N) New IR builder for spmw

[Fangtangtang]

Description

follows cornell-zhang#570.

This PR introduces a new (experimental) MLIR IR builder.

Design Overview

The builder adopts a two-phase design:

1. AST preprocessing (ast_preprocessor)
2. MLIR construction (ir_builder)

These two components are intended to serve as core infrastructure. They are expected to remain relatively stable over time and should not be modified frequently once the design matures.

In contrast, the builtin/ directory contains components related to syntax and semantic extensions. This part is designed to be highly extensible and is expected to evolve rapidly.
Note that some of the existing implementations under builtin/ are not yet fully tested, they primarily serve as current feature support and examples for future semantic extensions.

For more details, please refer to the README files in the corresponding directories.

Examples

New test cases are added under tests/exp/:

• tests/exp/basic/: basic syntax and usage examples. These tests are lowered to the CPU backend for functionality validation.

• tests/exp/spmw/: tests and examples for the new SPMW programming interface. (Currently without backend codegen or execution validation.)

Checklist

Please make sure to review and check all of these items:

• PR's title starts with a category (e.g. [Bugfix], [IR], [Builder], etc)
• All changes have test coverage (It would be good to provide ~2 different test cases to test the robustness of your code)
• Pass the formatting check locally
• Code is well-documented