You are close, but that isn’t quite right.
First, the file path should be assets/javascripts/lib/discourse-markdown/dropcaps.js.es6
Next it needs to have the contents of
export function setup(helper) {
helper.registerOptions((opts, siteSettings)=>{
opts.features['dropcaps'] = true; //!!siteSettings.dropcaps_enabled;
});
helper.whiteList([ 'span.dropcaps', 'span.sclass' ]);
}