Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

The 2.2.0 release for Mermaid introduced a breaking change to the type for Options in @docusaurus/mdx-loader.

This commit/line: 9c92a79#diff-545a046200b390be351720ea1e83af42ec7ed096a4351213383e1550467d8cdcR66

markdownConfig should be an optional type or have a default value assigned, to align with semver.

Reproducible demo

No response

Steps to reproduce

1. Use @docusaurus/mdx-loader in a plugin/app pre 2.2.0
2. Upgrade to 2.2.0
3. Observe: Build breaks with error:

[ERROR] TypeError: Cannot read properties of undefined (reading 'mermaid')
    at Object.mdxLoader (/Users/ben/flagsmith/docs/node_modules/@docusaurus/mdx-loader/lib/loader.js:115:43)
    at LOADER_EXECUTION (/Users/ben/flagsmith/docs/node_modules/loader-runner/lib/LoaderRunner.js:132:14)
    at runSyncOrAsync (/Users/ben/flagsmith/docs/node_modules/loader-runner/lib/LoaderRunner.js:133:4)
    at iterateNormalLoaders (/Users/ben/flagsmith/docs/node_modules/loader-runner/lib/LoaderRunner.js:250:2)
    at /Users/ben/flagsmith/docs/node_modules/loader-runner/lib/LoaderRunner.js:223:4
    at /Users/ben/flagsmith/docs/node_modules/webpack/lib/NormalModule.js:834:15
    at eval (eval at create (/Users/ben/flagsmith/docs/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:12:1)
    at processTicksAndRejections (node:internal/process/task_queues:83:21)
    at runNextTicks (node:internal/process/task_queues:65:3)
    at processImmediate (node:internal/timers:437:9)

Can be seen here: cloud-annotations/docusaurus-openapi#222

Expected behavior

Breaking changes should be reserved for major release cycles. markdownConfig should either be optional or have a default value assigned as its value.

Actual behavior

Build process breaks when upgrading to 2.2.0.

Your environment

• Docusaurus version used: v2.2.0
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Chrome Version 107.0.5304.87 (Official Build) (arm64), Node.js v18.11.0
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Mac OS 13.0

Self-service

• I'd be willing to fix this bug myself.