Convert to ESM.

Context: https://github.com/actions/typescript-action/pull/969

rollup.main.config.ts

@@ -0,0 +1,19 @@
+// See: https://rollupjs.org/introduction/
+
+import commonjs from '@rollup/plugin-commonjs'
+import json from '@rollup/plugin-json'
+import nodeResolve from '@rollup/plugin-node-resolve'
+import typescript from '@rollup/plugin-typescript'
+
+const config = {
+  input: 'src/main.ts',
+  output: {
+    esModule: true,
+    file: 'dist/main.js',
+    format: 'es',
+    sourcemap: false
+  },
+  plugins: [typescript(), nodeResolve({ preferBuiltins: true }), commonjs(), json()]
+}
+
+export default config