feat: exported Drop interface for #107

Deprecate snake_cased options and APIs, sed #109

test/unit/drop/drop.ts

@@ -0,0 +1,10 @@
+import { expect } from 'chai'
+import { Drop } from '../../../src/drop/drop'
+
+describe('drop/drop', function () {
+  class CustomDrop extends Drop { }
+
+  it('.valueOf() should return undefined by default', async function () {
+    expect(new CustomDrop().valueOf()).to.be.undefined
+  })
+})