(core) add missing tsconfig file that affects IDEs

Summary:
I missed committing a file that is important for editing files comfortably in the ext directory in an IDE. This diff:

 * Adds tsconfig-base-ext.json - that was the only intended change
 * Unrelated: Forces all creation of connections to the home db through a new `getOrCreateConnection` method which changes the `busy_timeout` if using Sqlite. This was an attempt to fix random "database is locked" test failures. I believe multiple connections to the home db as an sqlite file do not happen in self-hosted Grist (where there is a single node process) or in our SaaS (where the database is in postgres). It does affect Grist started using `devServerMain.ts` (where multiple processes accessing same database are started) or various test configurations when extra database connections are opened.
 * Unrelated: I added a `busy_timeout` for session storage, when it uses Sqlite. Again, I don't believe this affects self-hosted Grist or our SaaS.
 * Tweaked a `BillingDiscount` test that looked perhaps vulnerable to a stripe request stalling.

I can't be sure my tweaks actually help, since I didn't succeed in replicating the failures. Update: looks like the "locked" error can still happen :(

Test Plan: manual

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3450

app/server/lib/gristSessions.ts

@@ -73,8 +73,16 @@ function createSessionStoreFactory(sessionsDB: string): () => SessionStore {
       const store = new SQLiteStore({
         dir: path.dirname(sessionsDB),
         db: path.basename(sessionsDB),    // SQLiteStore no longer appends a .db suffix.
-        table: 'sessions'
+        table: 'sessions',
       });
+      // In testing, and monorepo's "yarn start", session is accessed from multiple
+      // processes, so could hit lock failures.
+      // connect-sqlite3 has a concurrentDb: true flag that can be set, but
+      // it puts the database in WAL mode, which would have implications
+      // for self-hosters (a second file to think about). Instead we just
+      // set a busy timeout.
+      store.db.run('PRAGMA busy_timeout = 1000');
+
       return assignIn(store, { async close() {}});
     };
   }