"Start new PWA" path separator fix for non-Windows platforms

Currently, the "Start new PWA" flow results in the following error on macOS:

```A system error occurred (ENOENT: no such file or directory, rename '/REDACTED\decompressedTemplate\pwa-starter-main' -> '/REDACTED\REDACTED')```

apps/pwabuilder-vscode/src/services/new-pwa-starter.ts

@@ -141,10 +141,10 @@ export function isNpmInstalled(): boolean {
 async function fetchFromGithub(): Promise<string> {
   const streamPipeline = promisify(pipeline);
 
-  const fetchedZipPath: string = `${repositoryParentURI?.fsPath}\\fetchedTemplate.zip`;
-  const decompressedZipPath: string = `${repositoryParentURI?.fsPath}\\decompressedTemplate`;
-  const decompressedRepoPath: string = `${decompressedZipPath}\\pwa-starter-main`;
-  const finalLocationPath: string = `${repositoryParentURI?.fsPath}\\${repositoryName}`;
+  const fetchedZipPath: string = `${repositoryParentURI?.fsPath}/fetchedTemplate.zip`;
+  const decompressedZipPath: string = `${repositoryParentURI?.fsPath}/decompressedTemplate`;
+  const decompressedRepoPath: string = `${decompressedZipPath}/pwa-starter-main`;
+  const finalLocationPath: string = `${repositoryParentURI?.fsPath}/${repositoryName}`;
 
   const res = await fetch(starterRepositoryURI);
   if(res.body) {
@@ -164,4 +164,4 @@ function inputCancelledWarning(): void {
 
 export function noNpmInstalledWarning(): void {
   vscode.window.showWarningMessage(noNpmWarning);
-}
+}