generatePkcePair() cannot run in browser environment

[kschiew]

Describe the bug
openid-client is a Nodejs library that is not mean to run in browser environments. Since it is technically possible that RP's generate their PKCE pairs on the frontend, our implementation of PKCE pair generation should not depend on openid-client.

To Reproduce

1. npm i @opengovsg/sgid-client in a React project.
2. import { generatePkcePair() } from @opengovsg/sgid-client in a .jsx/.tsx file.
3. Run const { codeVerifier, codeChallenge } = generatePkcePair()
4. App crashes, with the following error message in console:
   

Expected behavior
generatePkcePair() should run successfully and return <codeChallenge, codeVerifier> pair.

[kschiew]

As discussed in #52, the team currently does not endorse maintaining session details on the frontend, therefore as of now the team does not plan to support PKCE pair generation in the browser environment. If the user decides to do session management on frontend regardless, refer to https://www.npmjs.com/package/pkce-challenge.