MLE-24763 First batch of lint fixes

No functional changes. Rules enforced (changes made by Copilot):

- Use single quotes for strings.
- Use `const` wherever possible instead of `let` or `var`.

Author: rjrudin

eslint.config.js

@@ -35,6 +35,10 @@ module.exports = [
       "no-unused-vars": "error",
       "no-undef": "error",
 
+      // ES6+ modernization
+      "prefer-const": "error", // Use const for variables never reassigned
+      "no-var": "error", // Disallow var, use let/const instead
+
       // Spacing rules (disabled for initial setup - TODO: Fix in separate PR)
       "arrow-spacing": "off",
       "space-before-blocks": "off",

lib/basic-loader.js

@@ -33,7 +33,7 @@ function loadFile(callback, options) {
 
   const documentDescriptor = options.documentDescriptor;
   if (documentDescriptor === void 0 || documentDescriptor == null) {
-    throw new Error(`loadFile requires fileDescriptor option`);
+    throw new Error('loadFile requires fileDescriptor option');
   }
 
   fs.readFile(filePath, 'utf8', (err, data) => {
@@ -52,7 +52,7 @@ function loadFileStream(options) {
 
   const documentMetadata = options.documentMetadata;
   if (documentMetadata === void 0 || documentMetadata == null) {
-    throw new Error(`loadFileStream requires documentMetadata option`);
+    throw new Error('loadFileStream requires documentMetadata option');
   }
 
   let uriPrefix = options.uriPrefix;