fix: add skipLibCheck in tsconfig.tns.json

Currently when there is a web application and `ng add @nativescript/schematics` command is executed, the following error is thrown `ERROR in node_modules/@types/jasminewd2/index.d.ts(8,23): error TS2688: Cannot find type definition file for 'jasmine/v2'.`. Adding `skipLibCheck: true` will ensure that typescript will ignore `.d.ts` files from `node_modules`. As it requires too much efforts to add the option in `tsconfig.json`, we decided to add it inside `tsconfig.tns.json`.
As the option is added to `tsconfig.tns.json` file, there will be a difference compared to the behavior of `ng generate --collection=@nativescript/schematics [--shared]`:
* `ng generate --collection=@nativescript/schematics [--shared]` -> `skipLibCheck: true` is inside `tsconfig.json`
* `ng add @nativescript/schematics` -> `skipLibCheck: true` is inside `tsconfig.tns.json`

Rel to: #249

Author: Fatme

src/add-ns/_ns-files/tsconfig__nsext__.json

@@ -3,6 +3,7 @@
   "compilerOptions": {
     "module": "es2015",
     "moduleResolution": "node",
+    "skipLibCheck": true,
     "paths": {
       "@src/*": [
         "<%= sourceDir %>/*.tns.ts",