Drop detect-node dependency by coalescing eventsource inclusion

[Shaptic]

What

Simplifies usage of eventsource in the different environments:

• First, using ?? is much cleaner than the nonsense if (...defined...) cases
• Second, there's no need for isNode detection: we should just try to require it unconditionally if we can't find it somewhere in the globals or the window object

Historical context

• the global check was added in Check for a global EventSource before deciding what to use. #389 (cc @fnando)
• a react-native workaround was added in #760 - update EventSource definition for streaming operations in React Native #761 (cc @hunterpetersen)

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Why

Related: #824, since it drops the detect-node dependency.

Bundle size

Assets: 
  stellar-sdk.js (1.88 MiB)
  stellar-sdk.min.js (684 KiB)

[piyalbasu]

The old isNode package was checking to see if process is defined instead of checking if window is defined (which it looks like this solution is doing).

I've run into an issue in Freighter where I've tried to run stellar sdk in a service worker and because window wasn't defined, it falsely assumed I was in a node environment, which has led to some issues.

[piyalbasu]

Here's the PR that addressed the mentioned Freighter issue: stellar/js-stellar-base#567

It might not actually matter here, but thought I would mention

[Shaptic]

Good context, thanks! You're right that we're doing a window check, but I think it should still be okay here because the default logic flow stays the same. Can you double-check my rationale?

Previously:

1. is there an EventSource in the global variable? use it
2. is there a process (via isNode)? then require() it
3. is there a window in the global variable AND an EventSource on that window? use it
4. default to trying to require() it

Now, we drop (2) and move it to the "default case." I think this is fine, because if we're in Node, there won't be a window, and if there is a window, we can still use its EventSource if it has one. Then if all else fails, we try importing it.

[aristidesstaffieri]

awesome yeah I think the feature detection approach is better than env detection