[keyserver/landing] add @babel/plugin-transform-private-methods to list of plugins

Summary:
The day before the hackathon @ashoat and I were debugging an issue where the farcaster app signin redirect url was not being returned with the WebView. As we were working on a solution where we need to run yarn prod in landing + yarn prod-build && yarn prod in keyserver we were getting some babel issues that were resolved by adding the plugins in this diff

Linear task w/ discussion about the issue @ashoat and I were trying to solve: https://linear.app/comm/issue/ENG-7325/connect-to-farcaster-wo-siwn

Added the babel plugins + ran yarn cleaninstall

Made @ashoat blocking since this diff adds a dependency + new babel plugins

Depends on D11426

Test Plan: N/A + was able to test the entire SIWF flow during the hackathon + was able to run yarn prod in landing + yarn prod-build && yarn prod in keyserver w/o getting any errors

Reviewers: ashoat, atul

Reviewed By: ashoat, atul

Subscribers: tomek, ashoat

Differential Revision: https://phab.comm.dev/D11427