Core: On demand store

[tmeasday]

From the migration notes:

Introduces an opt-in feature flag, features.storyStoreV7, which loads stories in an "on demand" way (that is when rendered), rather than up front when the Storybook is booted. This way of operating will become the default in 7.0 and will likely be switched to opt-out in that version.

The key benefit of the on demand store is that stories are code-split automatically (in builder-webpack4 and builder-webpack5), which allows for much smaller bundle sizes, faster rendering, and improved general performance via various opt-in Webpack features.

The on-demand store relies on the "story index" data structure which is generated in the server (node) via static code analysis. As such, it has the following limitations:

• Does not work with storiesOf()
• Does not work if you used dynamic story names or component titles.

However, the autoTitle feature is supported.

Behavioral differences

The key behavioral differences of the v7 store are:

• SET_STORIES is not emitted on boot up. Instead the manager loads the story index independenly.
• A new event STORY_PREPARED is emitted when a story is rendered for the first time, which contains metadata about the story, such as parameters.
• All "entire" store APIs such as extract() need to be proceeded by an async call to loadAllCSFFiles() which fetches all CSF files and processes them.

Using the v7 store

To activate the v7 mode set the feature flag in your .storybook/main.js config:

module.exports = {
  // ... your existing config
  features: {
    storyStoreV7: true,
  },
};

NOTE: features.storyStoreV7 implies features.buildStoriesJson and has the same limitations.

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This PR is too large to be sensibly reviewed, instead it has been split into several smaller PRs that can be examined separately and changes will be cherry picked back here:

• Add @storybook/store and @storybook/preview-web #15999
• Manager changes for v7 on-demand store #16004
• Changes to builders for v7 store feature #16005
• Change ClientApi to be a facade in front of the store #16006
• Docs changes for on demand store #16009
• Addons and app changes for on demand store #16010
• Add modernInlineRendering feature #16011
• Add app layer presets #16012
• Addon-docs: Bugfixes for new on-demand architecture #16034

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Current limitations:

• HMR is not fully supported in the v7 store, adding/removing stories will not be picked up, you need to reload the page manually.
• v7 mode currently does not work with Storyshots
• v7 mode currently does not work with Composition
• v7 mode currently does not work with Links
• No UI is currently show for async story rendering (not necessarily a big problem but could be improved).

These limitations will be addressed in follow up PRs.

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