fix(@angular/cli): ignore unknown files when formatting schematic changes

When schematics modify files such as .gitignore, Prettier cannot infer a parser and formatting fails.

```
npx prettier --check .gitignore
[error] No parser could be inferred for file ".gitignore".
```

With ignore-unknown enabled, Prettier does not throw this parser error

Author: cexbrayat

packages/angular/cli/src/utilities/prettier.ts

@@ -44,7 +44,7 @@ export async function formatFiles(cwd: string, files: Set<string>): Promise<void
 
   await execFileAsync(
     process.execPath,
-    [prettierCliPath, '--write', '--no-error-on-unmatched-pattern', ...files],
+    [prettierCliPath, '--write', '--no-error-on-unmatched-pattern', '--ignore-unknown', ...files],
     {
       cwd,
       shell: false,