Add CJS support (#44)

Updated build workflow to generate CJS (CommonJS) files as well as ESM
(ECMAScriptModule).
This resolves #43.

build.config.js

@@ -0,0 +1,33 @@
+import { defineBuildConfig } from 'unbuild'
+
+export default defineBuildConfig({
+    clean: true,
+    declaration: true,
+    entries: [
+        './src/index.ts',
+        './src/constants.ts',
+        './src/lib/field-variant.ts',
+        './src/parser.ts',
+        './src/promise/index.ts',
+        './src/stream/index.ts'
+    ],
+    outDir: 'dist',
+    rollup: {
+        emitCJS: true,
+        esbuild: {
+            // reduce the overall package size by minifying the generated code.
+            // this isn't a "perfect" solution,
+            // but unless someone really wants to see the actual code
+            // (which they can by simply visiting GitHub),
+            // this will save roughly 20%.
+            minify: true,
+            platform: 'neutral',
+            target: 'node16',
+            treeShaking: true
+        },
+        output: {
+            exports: 'named',
+            generatedCode: 'es2015'
+        }
+    }
+})