fix(dev-server-storybook): rename storybook config file

[samvimes01]

Why I did this change

I had a bug and couldn't start storybook in my package with type: module

Steps to reproduce:

1. start a package with packcage.json "type: module" (or for example clone or download https://github.com/lit/lit-element-starter-ts)
2. according to the current guide add the dev-server-storybook

• create a .storybook/main.js with exports.default = (N.B. - it's the source of issue)
• create a .storybook/server.config.mjs

3. run wds --config .storybook/server.config.mjs
4. got the error ERR_REQUIRE_ESM require() of ES modules is not supported

What I did

So the fastest and easiest fix that works both in commonjs and module packages - rename main.js to main.cjs

1. renamed hardcoded filename in packages/dev-server-storybook/src/shared/config/readStorybookConfig.ts
2. changed docs and guide - rename main.js to main.cjs

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 276c50d4e6afc895b0b9fa4dc0dadbff5f868c78

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@web/dev-server-storybook	Minor

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[LarsDenBakker]

This is a breaking change that will break existing projects. It would be better to check if the .cjs file exists, and otherwise use the .js file.