Modify the tsconfig.json in lib/vscode/src/build.

This adds the flag skipLibCheck: true to tell TypeScript
to not type-check the declaration files at build time.

We need to add this because otherwise it checks the declaration
files and reports an error of duplicate type definitions
because we use Jest for our tests and they use Mocha and they
both use the global namespace "test" in their .d.ts files.