refactor: derive and reuse isWebpack5 in index loader

Agent-Logs-Url: https://github.com/TypeStrong/ts-loader/sessions/45ce3f0d-3ab9-47ff-8520-23271ab092af

Co-authored-by: johnnyreilly <1010525+johnnyreilly@users.noreply.github.com>

Author: Copilot

src/index.ts

@@ -41,7 +41,8 @@ function loader(
 ) {
   this.cacheable && this.cacheable();
   const callback = this.async();
-  const options = getLoaderOptions(this, typeof this.getOptions === 'function');
+  const isWebpack5 = typeof this.getOptions === 'function';
+  const options = getLoaderOptions(this, isWebpack5);
   const instanceOrError = getTypeScriptInstance(options, this);
 
   if (instanceOrError.error !== undefined) {