fix(router-store): use non-const enum to allow isolatedModules tsconfig option

[dangrussell]

Using a const enum here triggers "Cannot access ambient const enums when 'isolatedModules' is enabled." error when "isolatedModules" is true in tsconfig.json.

"isolatedModules" is set to true by default in new Angular CLI applications.

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[markostanimirovic]

Thanks @dangrussell!