feat: add miette-based diagnostic renderer with Rust integration

[idriss-hamadi]

Overview

Adds a new high-quality diagnostic renderer using the Rust miette crate, providing beautiful, user-friendly error messages with source code highlighting and precise error location indicators.

closes #968

Features

• Rich diagnostic rendering with Unicode/ASCII themes
• Source code highlighting with precise span annotations
• ANSI color support with fallback to ASCII theme
• Multiple severity levels (ERROR, WARNING, ADVICE)
• Hierarchical diagnostics supporting parent/child relationships
• Configurable output (context lines, custom footers)
• Graceful fallback when Rust extension unavailable

Implementation

• Rust extension (miette_py) using PyO3 bindings
• Python wrapper (diagnostic_miette.py) for seamless integration
• Maturin build system for mixed Python/Rust package compilation
• Comprehensive test suite (13 tests, 100% pass rate)

All quality checks pass

ruff format ruff check mypy pytest

13/13 tests passing with comprehensive coverage:

• Direct Rust function calls
• Python wrapper integration
• Source code + label rendering
• Multiple severity levels
• Hierarchical diagnostics
• Theme switching (Unicode/ASCII)
• Configurable options

[mark-koch]

Hi! Thanks for your contribution!

It looks like your code currently doesn't build, see this CI run:

  × Failed to build `guppylang @ file:///home/runner/work/guppylang/guppylang`
  ├─▶ The build backend returned an error
  ╰─▶ Call to `maturin.build_editable` failed (exit status: 1)

      [stdout]
      Running `maturin pep517 build-wheel -i
      /home/runner/work/_temp/setup-uv-cache/builds-v0/.tmptvXFzp/bin/python
      --compatibility off --editable`

      [stderr]
      error: current package believes it's in a workspace when it's not:
      current:
      /home/runner/work/guppylang/guppylang/guppylang/miette-py/Cargo.toml
      workspace: /home/runner/work/guppylang/guppylang/Cargo.toml

      this may be fixable by adding `guppylang/miette-py` to
      the `workspace.members` array of the manifest located at:
      /home/runner/work/guppylang/guppylang/Cargo.toml
      Alternatively, to keep it out of the workspace, add the package to the
      `workspace.exclude` array, or add an empty `[workspace]` table to the
      package's manifest.
      💥 maturin failed
        Caused by: Cargo metadata failed. Does your crate compile with `cargo
      build`?
        Caused by: `cargo metadata` exited with an error:
      Error: command ['maturin', 'pep517', 'build-wheel', '-i',
      '/home/runner/work/_temp/setup-uv-cache/builds-v0/.tmptvXFzp/bin/python',
      '--compatibility', 'off', '--editable'] returned non-zero exit status 1

      hint: This usually indicates a problem with the package or the build
      environment.

Could you fix that, so I can have a closer look and test locally? Try to follow the setup from the existing validator and execute_llvm maturin projects in the repo.