Use appropriate equality operator in changelog script

It is considered good practice to use JavaScript's type-safe strict equality operator === instead of the equality
operator ==. Compliance with this practice is enforced by the project's ESLint configuration, via the "eqeqeq" rule.

The script used to generate the changelog for Arduino IDE's auto-update dialog contained an inappropriate usage of the
equality operator. This caused linting runs to fail:

arduino-ide-extension/scripts/compose-changelog.js
  37:19  error  Expected '===' and instead saw '=='  eqeqeq

arduino-ide-extension/scripts/compose-changelog.js

@@ -34,7 +34,7 @@
   }, '');
 
   const args = process.argv.slice(2);
-  if (args.length == 0) {
+  if (args.length === 0) {
     console.error('Missing argument to destination file');
     process.exit(1);
   }