Improve handling of environment variables

[fionnachan]

Let's try out https://github.com/evanshortiss/env-var, and if we find it useful, we can do the same in other repos.

We should load all environment variables in a single file, then export them, so we don't have to call env-var each time an environment variable is used. The variables that are required for running the app should be marked as .required and the app should throw in case they are missing, one such example being NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID.

This would also allow us to get rid of https://github.com/OffchainLabs/arbitrum-token-bridge/blob/master/packages/arb-token-bridge-ui/src/util/index.ts#L24

Some env-var specifics for Next.js: evanshortiss/env-var#162 (comment)

[abhirajthakur]

Hey @spsjvc, I want to start contributing to the open source projects. I am new to it but I want to learn and contribute. Can I ask for the assignment of this issue. I am still new to this so I will be asking a lot of questions. Will it be ok?

[spsjvc]

Hey @abhirajthakur, and thank you for your interest in contributing! 💙

We're actually gonna handle this issue within the team, as we want to try out a couple of different solutions and unify the way we manage environment variables across multiple repositories.

Stay on the lookout for other issues, though!