[dev] Replace istanbul with nyc

As the istanbul package is not longer maintained, we need to migrate to nyc.
Currently istanbul doesn't work for the following code:

const { stdio = "inherit" } = options;

In this case grunt test command fails with an exception:

__cov_gCRxMpJElFAq6oDSOHYQNQ.s['1']++;var __awaiter=(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][0]++,this)&&(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][1]++,this.__awaiter)||(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['1'][2]++,function(thisArg,_arguments,P,generator){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['1']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['2']++;return new((__cov_gCRxMpJElFAq6oDSOHYQNQ.b['2'][0]++,P)||(__cov_gCRxMpJElFAq6oDSOHYQNQ.b['2'][1]++,P=Promise))(function(resolve,reject){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['2']++;function fulfilled(value){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['3']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['4']++;try{__cov_gCRxMpJElFAq6oDSOHYQNQ.s['5']++;step(generator.next(value));}catch(e){__cov_gCRxMpJElFAq6oDSOHYQNQ.s['6']++;reject(e);}}function rejected(value){__cov_gCRxMpJElFAq6oDSOHYQNQ.f['4']++;__cov_gCRxMpJElFAq6oDSOHYQNQ.s['8']++;try{__cov_gCRxMpJElFAq6oDSOHYQNQ.s['9']++;step(generator['throw'](value));}catch(e){__cov_gCRxMpJElFAq6oDSO

SyntaxError: Unexpected token :
    at new Script (vm.js:79:7)
    at createScript (vm.js:251:10)
    at Object.runInThisContext (vm.js:303:10)
    at Module._compile (internal/modules/cjs/loader.js:656:28)
    at Object.Module._extensions.(anonymous function) [as .js] (/Users/havaluova/Work3/nativescript-cli/node_modules/istanbul/lib/hook.js:107:24)
    at Module.load (internal/modules/cjs/loader.js:598:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:537:12)
    at Function.Module._load (internal/modules/cjs/loader.js:529:3)
    at Module.require (internal/modules/cjs/loader.js:636:17)
    at require (internal/modules/cjs/helpers.js:20:18)

It seems that when the above mentioned error occurs, istanbul exits with status code 0 and this is the reason why jenkins build doesn't fail in such situation.

Start with the grunt test command and find the Istanbul dependency and coverage configuration used by the CLI. Check how the Jenkins build receives test failures and verify the migration against the destructuring example that currently triggers a syntax error. Done means tests run with nyc and coverage errors produce a nonzero status.