fix: allow JS snapshots within TS plugin

So far we've always assumed that a Svelte file is a TS file for simplicity, due to our default language (which was removed a long time ago) and also because IIRC there were issues with TS having its snapshot scriptKind being switched. That seems to be no longer the case, and so we can get better at properly analyzing whether or not this is a TS file, to allow JSDoc to take action when it's a JS file.

#2555

packages/typescript-plugin/src/svelte-snapshots.ts

@@ -16,13 +16,18 @@ export class SvelteSnapshot {
         private svelteCode: string,
         private mapper: SourceMapper,
         private logger: Logger,
-        public readonly isTsFile: boolean
+        public isTsFile: boolean
     ) {}
 
-    update(svelteCode: string, mapper: SourceMapper) {
+    update(svelteCode: string, mapper: SourceMapper, isTsFile: boolean) {
         this.svelteCode = svelteCode;
         this.mapper = mapper;
         this.lineOffsets = undefined;
+        this.isTsFile = isTsFile;
+        // @ts-expect-error
+        this.scriptInfo!.scriptKind = this.isTsFile
+            ? this.typescript.ScriptKind.TS
+            : this.typescript.ScriptKind.JS;
         this.log('Updated Snapshot');
     }
 
@@ -107,7 +112,9 @@ export class SvelteSnapshot {
 
     setAndPatchScriptInfo(scriptInfo: ts.server.ScriptInfo) {
         // @ts-expect-error
-        scriptInfo.scriptKind = this.typescript.ScriptKind.TS;
+        scriptInfo.scriptKind = this.isTsFile
+            ? this.typescript.ScriptKind.TS
+            : this.typescript.ScriptKind.JS;
 
         const positionToLineOffset = scriptInfo.positionToLineOffset.bind(scriptInfo);
         scriptInfo.positionToLineOffset = (position) => {
@@ -366,7 +373,17 @@ export class SvelteSnapshotManager {
                 } else if (isSvelteFilePath(path)) {
                     this.logger.debug('Read Svelte file:', path);
                     const svelteCode = readFile(path) || '';
-                    const isTsFile = true; // TODO check file contents? TS might be okay with importing ts into js.
+                    const version = this.svelteCompiler?.VERSION ?? '4.2.0'; // built-in one is 4.x
+                    const isTsFile =
+                        // At some point we supported a default language of TS, but that was removed.
+                        // Assume that people using Svelte 5 are always using the attribute
+                        Number(version.split('.')[0]) >= 5
+                            ? svelteCode.includes('lang="ts"') ||
+                              svelteCode.includes('lang=ts') ||
+                              // Additional measures to prevent each file to start out with JS and then switch to TS
+                              !svelteCode.includes('<script') ||
+                              !svelteCode.includes('@type')
+                            : true;
                     let code: string;
                     let mapper: SourceMapper;
 
@@ -378,7 +395,7 @@ export class SvelteSnapshotManager {
                             typingsNamespace: this.svelteOptions.namespace,
                             // Don't search for compiler from current path - could be a different one from which we have loaded the svelte2tsx globals
                             parse: this.svelteCompiler?.parse,
-                            version: this.svelteCompiler?.VERSION
+                            version
                         });
                         code = result.code;
                         mapper = new SourceMapper(result.map.mappings);
@@ -410,7 +427,7 @@ export class SvelteSnapshotManager {
                 const canonicalFilePath = this.projectService.toCanonicalFileName(path);
                 const existingSnapshot = this.snapshots.get(canonicalFilePath);
                 if (existingSnapshot) {
-                    existingSnapshot.update(svelteCode, mapper);
+                    existingSnapshot.update(svelteCode, mapper, isTsFile);
                 } else {
                     this.snapshots.set(
                         canonicalFilePath,