feat!: Custom fetch support, remove node-fetch, ESM+CJS dual build, migrate to vitest, TS fixes, test improvements

[rolodato]

Apologies for this cursed PR - it snowballed from trying to get the fix added by #161 to typecheck.

TL;DR changes in this PR:

• Remove node-fetch and support passing in a custom fetch implementation
• Support abort signal in fetch calls, provided by feat: support abort signal for fetch calls #161
• Migrate codebase to ESM, but also build CJS for backwards compatibility
• Migrate from jest to vitest
• Fix environment document always being fetched twice immediately on startup, supersedes fix: env document fetched twice on startup #147
• Remove almost all @ts-ignores

node-http only supports signal in v3, which only supports ESM. Instead of bothering with node-http breaking changes and incompatibility between node-http and built-in fetch types, I removed it altogether in favour of built-in fetch. This also simplifies tests by explicitly passing in a mocked fetch implementation and not needing to mock globals, which doesn't play nice with TS.

By depending on built-in fetch and migrating to ESM, we're now requiring Node >= 18, which is enforced in engines in package.json. The bulk of the changes to imports are for making file extensions explicit, which are mandated by ESM.

Building CJS in addition to ESM lets us continue supporting use of require, which we're still using in our docs. The dual-build ESM+CJS approach is the same that https://github.com/auth0/node-auth0 is using, which I used as a reference.

There are no user-facing API breaking changes in this PR, but the removal of node-fetch and requirement of Node >= 18 might warrant a major version bump. I would like to release this as a v4 beta and get some real-world testing.

There's 1 minor bugfix in sdk/polling_manager.ts. We were fetching the environment document twice immediately on startup. This extra call has been removed.

We're also now down to just 1 @ts-ignore in the whole project.

[khvn26]

Approving this with two notions:

• We'll be major-versioning the SDK release including this due to new Node requirement.
• We'll perform a beta release.