Register Handlebars customizations

By default, builtin helpers and open-source Handlebars helpers are available. If these don't fit your requirements, you can still create your own custom helper or partial that you can use in your HTML templates or layout fields (ex: 'Cards' or 'Details List' layouts).

To avoid any conflict, each Web Part instance gets its own Handlebars isolated namespace (i.e. using Handlebars.create()) meaning registering customizations in the global Handlebars namespace won't work (ex: using Handlebars.registerHelper() directly).

To register a new Handlebars customization for the targeted Web Part (i.e. the Web Part instances where the extensibility library is registered and enabled):

1. In the library main entry point (i.e. the class implementing the IExtensibilityLibrary in interface), register your customization using the registerHandlebarsCustomizations() method. The namespace parameter corresponds to the targeted Web Part Handlebars isolated namespace
2. From here, use the Handlebars API to add your customizations to this specific namespace. They will be availabe in templates for registered Web Part instances:

   public registerHandlebarsCustomizations(namespace: typeof Handlebars) {
   
       // Register custom Handlebars helpers
       // Usage {{myHelper 'value'}}
       namespace.registerHelper('myHelper', (value: string) => {
           return new namespace.SafeString(value.toUpperCase());
       });
   }