perf: cache files during emitDts (#2701)

closes #2691

Author: dummdidumm

packages/svelte2tsx/src/emitDts.ts

@@ -277,7 +277,7 @@ interface SvelteMap {
  * those transformed source later on.
  */
 async function createSvelteMap(config: EmitDtsConfig): Promise<SvelteMap> {
-    const svelteFiles = new Map();
+    const svelteFiles = new Map<string, { transformed: string; isTsFile: boolean }>();
 
     // TODO detect Svelte version in here and set shimsPath accordingly if not given from above
     const noSvelteComponentTyped = config.svelteShimsPath
@@ -286,6 +286,12 @@ async function createSvelteMap(config: EmitDtsConfig): Promise<SvelteMap> {
     const version = noSvelteComponentTyped ? undefined : '3.42.0';
 
     function add(path: string): boolean {
+        const normalizedPath = path.replace(/\\/g, '/');
+
+        if (svelteFiles.has(normalizedPath)) {
+            return svelteFiles.get(normalizedPath)!.isTsFile;
+        }
+
         const code = ts.sys.readFile(path, 'utf-8');
         const isTsFile = /<script\s+[^>]*?lang=('|")(ts|typescript)('|")/.test(code);
         const transformed = svelte2tsx(code, {
@@ -295,13 +301,13 @@ async function createSvelteMap(config: EmitDtsConfig): Promise<SvelteMap> {
             version,
             noSvelteComponentTyped: noSvelteComponentTyped
         }).code;
-        svelteFiles.set(path.replace(/\\/g, '/'), transformed);
+        svelteFiles.set(normalizedPath, { transformed, isTsFile });
         return isTsFile;
     }
 
     return {
         add,
-        get: (key: string) => svelteFiles.get(key.replace(/\\/g, '/'))
+        get: (key: string) => svelteFiles.get(key.replace(/\\/g, '/'))?.transformed
     };
 }