fix: inconsistent MathML rendering & erroneous <br />s being added

[NicolasNewman]

📑 Summary

1. Resolves rendering differences caused by different default OS fonts & MathML implementations within browsers (See Math is occasionally rendered incorrectly (e.g., fraction inside sqrt) #5435)
2. Fixed some equations being cut-off in flowcharts

Resolves #5435

📏 Design Decisions

1)

As mentioned in the PR, differences between the default fonts for each OS was not taken into account when I implemented the useLegacyMathML config option. As a result, there is currently no way to force Mermaid to fallback to using KaTeX's stylesheets if they are provided. To allow this to be done, I added an additional config option forceLegacyMathML to explicitly tell Mermaid to default to CSS-based rendering.

Ideally this should be changed to a single config option that takes one of 3 values but I don't know what the protocol for deprecating config options are.

2)

While fixing the above I noticed the spacing for some equations in flowchart vertices were incorrect. This was due to an erroneous <br /> tags being added by the regex replacement vertexText.replace(/\\n|\n/g, '<br />') being triggered on unintended line breaks in the template string in renderKatex().

Before:

After:

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🔖 targeted develop branch

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 5.72%. Comparing base (e68125e) to head (22bd262).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #5483      +/-   ##
==========================================
- Coverage     5.73%   5.72%   -0.01%     
==========================================
  Files          277     278       +1     
  Lines        41999   42012      +13     
  Branches       515     490      -25     
==========================================
  Hits          2407    2407              
- Misses       39592   39605      +13     

Flag	Coverage Δ
unit	5.72% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
packages/mermaid/src/diagrams/common/common.ts	53.01% <0.00%> (-0.28%)	⬇️

... and 1 file with indirect coverage changes