Switch from Gulp file to build script; update Babel and Jest

.babelrc

@@ -0,0 +1,4 @@
+// Transpile for current Node environment by default (Jest, babel-node)
+{
+  "presets": ["@babel/preset-env"]
+}

build.js

@@ -0,0 +1,56 @@
+/* eslint-disable import/no-extraneous-dependencies, no-console */
+import * as babel from "@babel/core";
+import { promises as fs } from "fs";
+import { glob } from "glob";
+
+(async function main() {
+  // Clear the build folder
+  await fs.rm("./build", { recursive: true, force: true });
+  await fs.mkdir("./build");
+
+  // Copy files to build folder
+  await Promise.all(
+    ["package.json", "LICENSE", "README.md"].map(file =>
+      fs.copyFile(file, `build/${file}`)
+    )
+  );
+
+  // Transpile for Node (except the browser entrypoint)
+  // In Babel, setting sourceMaps: true seems like it should produce external
+  // source maps and link them:
+  // https://babeljs.io/docs/options#source-map-options
+  // But it doesn't:
+  // https://github.com/babel/babel/issues/5261
+  // You can only produce external sourcemaps using the CLI, so I write
+  // the maps to file myself and append a reference in the transpiled source
+  const srcFiles = await glob("*.js", { cwd: "src" });
+  await Promise.all(
+    srcFiles.map(file =>
+      (async () => {
+        console.log(`Transpiling ${file}`);
+
+        // Transpile and produce source maps
+        const transpile = await babel.transformFileAsync(`src/${file}`, {
+          presets: [["@babel/preset-env"]],
+          plugins: ["add-module-exports"], // No .default()
+          sourceMaps: true,
+          sourceRoot: "../src"
+        });
+
+        // Write transpiled source
+        await fs.writeFile(
+          `build/${file}`,
+          `${transpile.code}\n//# sourceMappingURL=${file}.map\n`
+        );
+
+        // Write source maps
+        await fs.writeFile(
+          `build/${file}.map`,
+          JSON.stringify(Object.assign(transpile.map, { file }))
+        );
+      })()
+    )
+  );
+
+  console.log("Done");
+})();