Settings rollup uses tsc folder now.

Seems to be only causing a problem on MacOS. On other platforms rollup
finds our files, but on MacOS it doesn't:

[!] Error: Could not resolve './brave_overrides/index.js' from gen/chrome/browser/resources/settings/tsc/settings.js

Upstream is picking up files from tsc folder now for rollup, so it seems
that's where we should drop ours.

Chromium change:

https://source.chromium.org/chromium/chromium/src/+/36d4bff7cb074ec08a3097f0b05eb375bfd04087

Author: dpapad <dpapad@chromium.org>
Date:   Fri Jul 30 09:23:37 2021 +0000

    Settings: Migrate to TypeScript compiler, still using JS.

    Bug: 1234307

browser/resources/settings/BUILD.gn

@@ -17,7 +17,7 @@ import("//tools/polymer/polymer.gni")
 import("//ui/webui/resources/tools/generate_grd.gni")
 import("//ui/webui/resources/tools/js_modulizer.gni")
 
-preprocess_folder = "preprocessed"
+preprocess_folder = "tsc"
 preprocess_manifest = "brave_preprocessed_manifest.json"
 preprocess_gen_manifest = "brave_preprocessed_gen_manifest.json"