feat(core): support docusaurus.config.cjs as default file name

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Related: #6520, #5379

I have heard of users who have "type": "module" in their package.json, which often prevents the docusaurus.config.js file from being read correctly. We should support .cjs extension out-of-the-box, which can also enable us to support .mjs, or even .ts in the future.

In addition, I fixed a little bug with the error message where it's using the default config file name instead of the actual one.

Test Plan

Added a test case.

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 69	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 79	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 796 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	598 kB
website/build/index.html	38.9 kB

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[IvanGoncharov]

I discovered that issue trying to migrate graphql-js to ESM and docusaurus is the only blocker left: graphql/graphql-js#3564
@Josh-Cena Big thanks for this PR 👍

@slorber @lex111 Can you please release a new beta with this change? 🙏

[Josh-Cena]

@IvanGoncharov This is more complex than that. See #7379 also, which kind of got blocked because of a cryptic error I don't know how to fix for now (and haven't looked deeply into).

If you just want the changes from this PR, you can always use docusaurus start --config docusaurus.config.cjs.

Also, we have our release schedule, so if we reviewed and merged it, you can always use the canary release instead of waiting for the next stable release.

[Josh-Cena]

Related: #7379

[slorber]

LGTM 👍