feat: add assertEnv - #19

Adds assertEnv to @plexus-ms/std: read the first environment variable that is set from a list of candidate names, or throw.

const secret = assertEnv('AUTH_SECRET', 'BETTER_AUTH_SECRET', { hint: 'use `pnx auth secret` to generate' });
const baseURL = assertEnv('APP_URL', 'AUTH_URL', 'BETTER_AUTH_URL', 'VERCEL_URL');

Names are tried in order. A variable that is unset or blank counts as missing and the returned value is trimmed, so AUTH_SECRET= fails loudly instead of handing back an empty secret.

Error messages name every candidate, Oxford-comma style, with the hint appended after a colon:

Missing environment variable AUTH_SECRET
Missing environment variable AUTH_SECRET or BETTER_AUTH_SECRET: use `pnx auth secret` to generate
Missing environment variable APP_URL, AUTH_URL, BETTER_AUTH_URL, or VERCEL_URL

Two overloads type the trailing options object and require at least one name, so assertEnv() and assertEnv({ hint }, 'A') are both compile errors.

Note: the package had no ambient types at all, so this adds @types/node and an explicit types: ["node"] for process.env.

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