feat: monorepo setup — npm workspaces scaffolding for package split

[RobertLD]

Summary

Prerequisite for #488. Convert the libscope repo to an npm workspaces monorepo so that sub-packages can be developed, built, and published independently from a single repository.

Problem / Motivation

Before any individual package can be extracted (#488), the repo needs the workspaces infrastructure in place. Without it, cross-package imports won't resolve during development and CI won't know how to build each package independently.

Proposed Solution

• Add an npm workspaces config to the root package.json pointing at each packages/* directory
• Create a packages/ directory and stub out a package.json for each planned sub-package
• Set up a root-level tsconfig.json with project references so tsc can build packages in dependency order
• Configure CI to run lint, typecheck, test, and build across all workspaces
• Shared dev tooling (eslint, prettier, vitest config) stays at the root and is inherited by packages

Acceptance Criteria

• Root package.json has a workspaces field covering all packages/* entries
• npm install from the root installs all workspace dependencies and symlinks cross-package deps correctly
• Each sub-package has a stub package.json with correct name, main, types, and exports fields
• npm run build from the root builds all packages in dependency order without errors
• CI runs lint, typecheck, test, and build for each workspace on Node 20 and 22
• Existing libscope CLI and all current tests continue to pass unchanged

Out of Scope

• Actually moving code into packages — that happens in the individual package extraction issues
• Changing any public API surface
• Turborepo or other build orchestration beyond npm workspaces (can be added later if build times warrant it)

Technical Notes

• npm workspaces requires npm ≥7 / Node ≥15; CI already runs Node 20+, so no constraint there
• TypeScript project references (composite: true, references: [] in tsconfig) will be needed per package for incremental builds
• The existing root tsconfig.json should become the base config; each package extends it