[tsp] Adding empty project for the TypeSpec library.

[glecaros]

This pull request introduces the initial setup for the new @alloy-js/typespec package, including its configuration, dependencies, and build/test scripts. The changes ensure the package is ready for development, testing, and documentation generation, and integrate it into the project’s monorepo structure.

New package setup and configuration:

• Added packages/typespec/package.json to define the package name, exports/imports, scripts, dependencies, and devDependencies for @alloy-js/typespec.
• Added packages/typespec/tsconfig.json to configure TypeScript compilation settings, declaration output, project references, and file inclusions/exclusions.
• Added packages/typespec/api-extractor.json to configure API Extractor for documentation generation.
• Added packages/typespec/vitest.config.ts to set up Vitest testing configuration and integrate the Alloy rollup plugin.

Monorepo integration:

• Updated pnpm-lock.yaml to register @alloy-js/typespec and its dependencies in the workspace, ensuring proper package management and linking.

[pkg-pr-new]

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npm i https://pkg.pr.new/alloy-framework/alloy/@alloy-js/typespec@318

commit: dbf58c5