Emit ESM and CJS, built with rollup (#121)

* Build with rollup

* Add a build step

* Add a tsconfig.build

* Use `NodeNext` rather than `nodenext`.

* Add `module` and `typings` properties

.github/workflows/push.yaml

@@ -21,6 +21,8 @@ jobs:
               run: npm install
             - name: lint
               run: npm run lint
+            - name: build
+              run: npm run build
             - name: compile typescript
               run: npx tsc
             - name: test

CHANGELOG.md

@@ -7,6 +7,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [20.0.0]
+
+### Changed
+
+-   We now emit CommonJS and ESM modules. The ESM and CJS are now found, respecively, in the `dist/esm` and `dist/cjs` directories.
+
+## [19.0.0]
+
+### Changed
+
+-   Updated dependencies. No user-visible changes are expected.
+
 ## [18.0.0]
 
 ### Removed