[keyserver] Add natural library for stemming and removing stopwords

Summary:
issue: https://linear.app/comm/issue/ENG-3169/write-a-query-that-allows-to-fetch-messages-that-match-a-certain-regex
natural library github, and yarn page

Test Plan: run yarn dev and yarn prod in keyserver/, checked that there are no errors. Also run the web app in prod mode and checked that it works.

Reviewers: ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7075