Fix npx availability check on Windows

[kraenhansen]

This fixes #77

Summary

• Run the components add npx --version availability check through the shell so Windows can resolve npx.cmd/npx.ps1
• Add coverage for the components add command spawn options

Verification

• npm test -- --runTestsByPath src/components/__tests__/add-command.test.ts
• npm run build
• npm test
• npm run lint (passes with existing warnings in src/__tests__/branches.test.ts and src/__tests__/casing.test.ts)