Switch to ESM for source and test, use CommonJS for dist (#806)

jest.config.mjs

@@ -0,0 +1,14 @@
+import { createDefaultEsmPreset } from "ts-jest";
+
+const esmPreset = createDefaultEsmPreset({
+  tsconfig: "./tsconfig.json",
+});
+
+export default {
+  ...esmPreset,
+  clearMocks: true,
+  moduleFileExtensions: ["js", "mjs", "ts"],
+  testEnvironment: "node",
+  testMatch: ["**/*.test.ts"],
+  verbose: true,
+};