Be strict about env var names.

[ericsnowcurrently]

Currently dotenv.parse() allows environment variable names which aren't actually valid for environment variables. This change ensures that they are.

[maxbeatty]

Can you elaborate on what makes for a valid environment variable? process.env is just an object and that’s what we are trying to support populating.

If we are going to update the battle-tested regex, I’d like to see a lot of tests.

[ericsnowcurrently]

I was thinking of shells. If folks are using .env files for more than actual environment variables then yeah a broader regex makes sense. Regardless, I can filter bogus names after the fact.