Add support for both CJS/ESM plugins

[Harjun751]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Resolves #2867

Can consider closing #2851

Overview of changes:
(Scope of this PR shifted quite a bit, but ultimately it's still addressing the same )

Updates plugin loading logic to load .cjs/.esm files, allowing for more flexibility when mixing modules in the markbind plugin directory.

Adds a new test site to test out all possible configurations of scopes and plugin modules.

Adds documentation on how to manage the different module systems with plugins. Bump minor version of node in user guide.

Check out the deploy preview's "Writing Plugins" page for the changes

Details

Anything you'd like to highlight/discuss:

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)

Add support for both CJS/ESM plugins

Let's

• Update plugin loading to allow for .cjs/.mjs extensions
• Add a new test site to test custom plugin configurations
• Update documentation on writing custom ESM/CJS plugins
• Bump minor node version in user guide to
  allow for ESM loading using require

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

---

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

---

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.04%. Comparing base (b641bc9) to head (a698212).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
packages/core/src/plugins/PluginManager.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2876   +/-   ##
=======================================
  Coverage   71.04%   71.04%           
=======================================
  Files         131      131           
  Lines        7117     7119    +2     
  Branches     1683     1653   -30     
=======================================
+ Hits         5056     5058    +2     
- Misses       1961     2055   +94     
+ Partials      100        6   -94     

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[gerteck]

LGTM! thanks for the PR, just some notes or possible nits

[gerteck]

LGTM, just a comment on the comment

Also, in the PR description, could you add if applicable:

Breaking change release note preparation (if applicable):
To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:
what was the old feature that was made obsolete
any replacement feature (if any), and
how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

An example is in #2622

[Harjun751]

LGTM, just a comment on the comment

Also, in the PR description, could you add if applicable:

Breaking change release note preparation (if applicable):
To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:
what was the old feature that was made obsolete
any replacement feature (if any), and
how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

An example is in #2622

Just tested, there's no breaking change in v6.0 vs now 👍