Add support for "exports" field for extra languages

tools/lib/external_language.js

@@ -2,6 +2,7 @@ const fs = require("fs");
 const fsProm = require("fs").promises;
 const glob = require("glob-promise");
 const path = require("path");
+const { fileURLToPath, pathToFileURL } = require('url')
 
 const MODULE_DEFINER = /module\.exports\.definer\s*=/;
 
@@ -39,15 +40,32 @@ class LanguagePackage {
     }
   }
 
+  resolveExportsField(exports) {
+    if(typeof exports === 'string') return exports
+    if(Array.isArray(exports)) return exports[0];
+    if('.' in exports) return this.resolveExportsField(exports['.'])
+    if('default' in exports) return this.resolveExportsField(exports.default)
+    if('highlight-js' in exports) return this.resolveExportsField(exports['highlight-js'])
+    if('import' in exports) return this.resolveExportsField(exports.import)
+    if('require' in exports) return this.resolveExportsField(exports.require)
+  }
+
   // try to find a language module by probing package.json
   async tryPackageJSON() {
     const pack = path.join(this.dir, "package.json");
     if (fs.existsSync(pack)) {
       const data = await fsProm.readFile(pack);
       const json = JSON.parse(data);
-      if (json.main) {
-        this.type = "npm";
-        const file = path.join(process.cwd(), this.dir, json.main);
+      if (json.main || json.exports) {
+        const extraDir = path.join(process.cwd(), this.dir);
+        const file = json.main ?
+          require.resolve(extraDir) :
+          fileURLToPath(new URL(this.resolveExportsField(json.exports), pathToFileURL(extraDir)));
+
+        if(!file.endsWith('.js')) throw new Error('Extra language file must use .js extension');
+
+        this.type = json.type === "module" ? "npm-esm" : "npm-cjs";
+
         const content = await fsProm.readFile(file, { encoding: "utf8" });
         // many existing languages seem to export a `definer` function rather than
         // simply export the language module directly.  This checks for that and if