Investigate HMR

We should investigate HMR and possibly enable it by default at some point.

https://github.com/angular/angular-cli/wiki/stories-configure-hmr

Start with the linked HMR configuration guide and inspect the Ionic CLI's current development-server behavior. Determine whether enabling HMR by default is feasible, what configuration and compatibility questions remain, and document a decided scope for the follow-up work.

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