node: Support no-code `init` via `--import` or `--require` and env variables

[timfish]

Problem Statement

@sentry/node already supports much of the basic configuration via environment variables. These are great because you can change often change settings without re-deploying your code. However, users are still required to create a JavaScript file to call Sentry.init().

Solution Brainstorm

If we add an init export (@sentry/node/init) that calls Sentry.init() users can then initialise Sentry and get all the auto-instrumentation magic without writing any code:

> SENTRY_DSN=https://examplePublicKey@o0.ingest.sentry.io/0 node --import=@sentry/node/init app.mjs