build: upgrade ng-packagr for ivy

[paullryan]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• [N/A] Tests for the changes have been added (for bug fixes / features)
• [N/A] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

This uses ng-packagr >= 5.6.0 (using 5.7.0) which fixes some package linkage issues with ngcc to prepare for angular 9.x and ivy. Without it errors like the following occur on SSR builds with NGXS

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

ERROR in /Users/pryan/workspaces/@campgroundbooking/portal/node_modules/@ngxs/devtools-plugin/__ivy_ngcc__/bundles/ngxs-devtools-plugin.umd.js 4:192
Module parse failed: Unexpected character '@' (4:192)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
|     typeof exports === 'object' && typeof module !== 'undefined' ? factory(exports, require('@angular/core'), require('@ngxs/store'), require('rxjs/operators'),require('@angular/core'),require('@ngxs/store')) :
|     typeof define === 'function' && define.amd ? define('@ngxs/devtools-plugin', ['exports', '@angular/core', '@ngxs/store', 'rxjs/operators','@angular/core','@ngxs/store'], factory) :
>     (global = global || self, factory((global.ngxs = global.ngxs || {}, global.ngxs['devtools-plugin'] = {}), global.ng.core, global['ngxs-store'], global.rxjs.operators,global.ng.core,global.@ngxs/store));
| }(this, function (exports, core, store, operators,ɵngcc0,ɵngcc1) { 'use strict';
|

The problem is the improper global resolution of the UMD submodule (e.g. global.ng.core,global.@ngxs/store)

Issue Number: N/A

What is the new behavior?

Packaged builds will properly resolve these submodules for ngcc.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[markwhitfeld]

Thank you & congratulations on your first time contribution.
Looks good to me.