Add `spm doctor` that should report missing config or binaries

[DecampsRenan]

I want to be able to run spm doctor to detect missing binaries (yarn, npm, pnpm) and also to detect bad project config (package-lock.json and yarn.lock in the same project for example).

A spm doctor --fix should allow to fix fixable issues (remove unused lock file, remove node_modules and reinstall them with the correct package manager, etc..)

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Checks to perform

• Detected package manager is installed on the system
• Package manager version is compatible / up to date
• Lock file is consistent with package.json (no drift)
• node_modules exists and is not corrupted
• Node.js version satisfies engines field (if present)

Expected output

$ spm doctor
✔ Package manager: yarn (v4.1.0)
✔ Lock file: yarn.lock is consistent
✔ node_modules: present
✔ Node.js: v20.11.0 (satisfies >=18)

All checks passed!