fix(@schematics/angular): enable TypeScript skipLibCheck in new workspace

[alan-agius4]

This commit enables skipLibCheck in new workspaces which is now recommended by TypeScript. When enabled, type checking of declaration files is skipped as rather than doing a full check of all d.ts files, TypeScript will type check the code you specifically refer to in your app’s source code.

See: https://www.typescriptlang.org/tsconfig#skipLibCheck

Note: this option is also enabled by default when running tsc --init

npx tsc --init               

Created a new tsconfig.json with: TS 
  target: es2016
  module: commonjs
  strict: true
  esModuleInterop: true
  skipLibCheck: true
  forceConsistentCasingInFileNames: true

Some more context: microsoft/TypeScript-Website#970

[clydin]

Should we only do this in the build itself? I could see the potential errors being useful in the IDE.

[alan-agius4]

Should we only do this in the build itself? I could see the potential errors being useful in the IDE.

The LS will not report errors in DTS files.

[clydin]

In that case, i don't think there is a compelling reason not to enable it by default especially if tsc --init does so as well.

[dgp1130]

At some point we should figure out whether we want to migrate existing apps to enable skipLibCheck too, but that doesn't need to block this.

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