Make asinit exit if package.json has a "type" not equal to "module"

Fixes AssemblyScript/website#237. When the "type" field isn't specified,
or if package.json doesn't exist, it's already set to "module". This
removes the remaining hole when "type" is set to, say, "commonjs", by
refusing to continue.

Author: CountBleck

bin/asinit.js

@@ -115,12 +115,23 @@ const paths = [
   [gitignoreFile, "Git configuration that excludes compiled binaries from source control."],
   [asconfigFile, "Configuration file defining both a 'debug' and a 'release' target."],
   [packageFile, "Package info containing the necessary commands to compile to WebAssembly."],
-  [testsIndexFile, "Stater test to check that the module is functioning."],
+  [testsIndexFile, "Starter test to check that the module is functioning."],
   [indexHtmlFile, "Starter HTML file that loads the module in a browser."]
 ];
 
 const formatPath = filePath => "./" + path.relative(projectDir, filePath).replace(/\\/g, "/");
 
+if (fs.existsSync(packageFile)) {
+  const pkg = JSON.parse(fs.readFileSync(packageFile));
+  if ("type" in pkg && pkg["type"] !== "module") {
+    console.error(stdoutColors.red([
+      `Error: The "type" field in ${formatPath(packageFile)} is set to "${pkg["type"]}".`,
+      `       asinit requires the "type" field to be set to "module" (ES modules).`
+    ].join("\n")));
+    process.exit(1);
+  }
+}
+
 console.log([
   "Version: " + version,
   "",