CLI-less api for node

[igor-dv]

Issue: #4249

What I did

There are cases when people want to run storybook from their code.
I've exposed the "standalone" api, so now it will be possible to do something like this:

import build from '@storybook/react/standalone';

build({
  mode: 'dev',
  port: 9009,
  configDir: './storybook',
  frameworkPresets: [],
});

[codecov]

Codecov Report

Merging #4344 into master will decrease coverage by 0.42%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4344      +/-   ##
==========================================
- Coverage   40.04%   39.62%   -0.43%     
==========================================
  Files         503      519      +16     
  Lines        5903     5966      +63     
  Branches      795      805      +10     
==========================================
  Hits         2364     2364              
- Misses       3156     3209      +53     
- Partials      383      393      +10

Impacted Files	Coverage Δ
app/svelte/src/server/build.js	0% <ø> (ø)	⬆️
app/svelte/src/server/index.js	0% <ø> (ø)	⬆️
app/vue/src/server/index.js	0% <ø> (ø)	⬆️
app/html/src/server/index.js	0% <ø> (ø)	⬆️
lib/core/src/server/utils.js	36.36% <ø> (+7.79%)	⬆️
app/marko/src/server/index.js	0% <ø> (ø)	⬆️
app/mithril/src/server/index.js	0% <ø> (ø)	⬆️
app/polymer/src/server/index.js	0% <ø> (ø)	⬆️
app/react/src/server/index.js	0% <ø> (ø)	⬆️
app/ember/src/server/index.js	0% <ø> (ø)	⬆️
... and 37 more

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[igor-dv]

Will document it later 😇 I swear

[transitive-bullshit]

@igor-dv I wasn't aware this was so recent of an addition, but I've found it extremely useful.

Thanks!!