(core) Simple localization support and currency selector.

Summary:
- Grist document has a associated "locale" setting that affects how currency is formatted.
- Currency selector for number format.

Test Plan: not done

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D2977

app/common/DocumentSettings.ts

@@ -0,0 +1,4 @@
+export interface DocumentSettings {
+  locale: string;
+  currency?: string;
+}