[lint] Prettier: Ignore tuta-sdk build folder

This wasn't triggering any errors, but if you have anything in 'target'
due to building tuta-sdk previously, it would take an eternity to run
`style:fix`. Let's just not worry about that directory since it isn't
even checked in...

.prettierignore

@@ -9,3 +9,4 @@ app-android/
 app-ios/
 fdroid-metadata-workaround/
 packages/tutanota-crypto/lib/internal/
+tuta-sdk/rust/target