Core: Fix managerHead preset in main.ts

[ndelangen]

Closes #22597

What I did

I moved the code to load manager-head.html to a preset value, then get the value needed by the builder-manager via presrt.apply.
This allows the user to set this value via main.ts if they so desire.

I also added this as a valid presetProperty in StorybookConfig interface.

How to test

In a sandbox:

// main.ts
export default {
  //...
  managerHead: (head) => `
    ${head}
    <style id="hello">
        * {
            background: indianred;
        }
    </style>
 `
}

[shilman]

I added a "how to test" section to the description if we want to support the 6.5 API. The default implementation should read the value of manager-head.html if it exists and return its contents.

[ndelangen]

The default implementation should read the value of manager-head.html if it exists and return its contents.

That's what it does, of course..
Am I missing something?

[shilman]

@ndelangen it just returns the filename. It does not return the contents of the file. Pleas read my "how to test" addition to the PR description

[ndelangen]

Understood, I'll fix it.

[ndelangen]

ok, fixed, and used your steps on how to test: