Can't resolve all parameters for Custom in Angular v6.x

[surekhabedakihal]

Warning: Can't resolve all parameters for Custom in node_modules/wp-api-angular/Custom.d.ts: ([object Object], [object Object], ?). This will become an error in Angular v6.x

[s4m0r4m4]

I bumped the versions to Angular v11 and should have fixed the issues in this PR: #48