[keyserver] Add type workaround for Intl.Segmenter

Summary:
As discussed in D7077 we want to add type definition for Intl, because the type provided by flow doesn't include Segmenter field we need.
Here are the typescript types: https://github.com/microsoft/TypeScript/blob/main/src/lib/es2022.intl.d.ts

Test Plan: run flow in keyserver/. Checked that it is possible to use segment method of the returned object

Reviewers: ashoat, tomek, kamil

Reviewed By: ashoat

Subscribers: atul

Differential Revision: https://phab.comm.dev/D7238