Add CodSpeed performance benchmarks

[codspeed-hq]

Summary

• Added CodSpeed continuous performance benchmarking to track the TypeScript compiler's performance over time.
• Created 9 benchmarks covering the core compiler pipeline: scanner, parser, and transpile operations, each tested at small, medium, and large input sizes.
• Integrated with CI via a dedicated GitHub Actions workflow using CodSpeed's simulation mode for reproducible measurements.

Changes

• benches/compiler.bench.mts: Benchmark suite using vitest bench, exercising ts.createScanner, ts.createSourceFile, and ts.transpileModule with TypeScript source inputs of varying complexity.
• vitest.config.mts: Vitest configuration with the @codspeed/vitest-plugin for CodSpeed integration.
• .github/workflows/codspeed.yml: GitHub Actions workflow that builds the compiler and runs benchmarks on every push to main and on pull requests, using OIDC authentication.
• package.json / package-lock.json: Added vitest and @codspeed/vitest-plugin as dev dependencies.
• README.md: Added CodSpeed badge.

Next steps

• Merge this PR to establish baseline performance measurements on main.
• Future PRs will automatically receive performance reports comparing against the baseline.
• Additional benchmarks can be added to benches/ to cover more compiler operations (e.g., type checking, declaration emit).
• Visit the CodSpeed dashboard to explore results and configure performance checks.