[JS] Split out defineFlow into stream/non-stream and make flows callable as functions.

[apascal07]

Fixes #391.

Instead of:

const myFlow = defineFlow(...);
const output = await runFlow(myFlow, ...);

const myStreamingFlow = defineFlow(...);
const { output, stream } = await streamFlow(myStreamingFlow, ...);

for await (const chunk of stream()) {
  console.log(chunk);
}

console.log(await output());

You will now do:

const myFlow = defineFlow(...);
const result = myFlow(...);

const myStreamingFlow = defineStreamingFlow(...);
const { stream, output } = await myStreamingFlow(...);

// `stream` and `output` are now values, not functions.

for await (const chunk of stream) {
  console.log(chunk);
}

console.log(await output);

Checklist (if applicable):

• Tested (manually, unit tested, etc.)
• Changelog updated
• Docs updated

[pavelgj]

could you please summarize API changes in the PR description?

[apascal07]

could you please summarize API changes in the PR description?

Done.

[pavelgj]

ugh, I forgot about this PR... and now I have a change that rips our durable flows, that massively conflicts with this...

[pavelgj]

could you please summarize API changes in the PR description?

Done.

I don't remember, have we discussed this API at some point ([in]formal review with the team)?

[apascal07]

could you please summarize API changes in the PR description?

Done.

I don't remember, have we discussed this API at some point ([in]formal review with the team)?

Chris filed this issue (#391) with the resolution based on a discussion where at least you, me, and Michael were involved.

[pavelgj]

could you please summarize API changes in the PR description?

Done.

I don't remember, have we discussed this API at some point ([in]formal review with the team)?

Chris filed this issue (#391) with the resolution based on a discussion where at least you, me, and Michael were involved.

Ah, I found the discussion. I am a bit torn on splitting out defineFlow and defineStreamingFlow... but I think it might simplify a few thing... so, yeah, LGTM.