Server: Fix .stories.yml support

[shilman]

Closes #22896

What I did

Restore .yml support in @storybook/server:

• Add .yml to indexer
• Fix yaml-loader behavior for webpack (not sure what changed, but we need asJSON option now)
• Remove inDevelopment flag from server sandbox to improve DX

Self-merging @ndelangen @tmeasday

How to test

In the internal/server-webpack5 sandbox:

1. Update main.js to include .stories.yml files:

export default { stories: ['../stories/**/*.mdx', '../stories/**/*.stories.@(json|yml)'], ... }

2. Add a file stories/button.stories.yml with the contents:

title: Example/ButtonYML
parameters:
  server:
    url: https://storybook-server-demo.netlify.app/api
    id: button
args:
  label: ButtonYML
stories:
  - { name: Primary, args: { primary: true } }

3. View the new story in dev/prod modes