feat(MathJax): Add support for MathJax extension, e.g. physics package

[huanyushi]

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (refactoring and improving code)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Description

MathJax provides many extensions that help us input mathematical expressions more conveniently. Among them, the physics package is particularly important for STEM majors, as it allows us to easily input matrices, Dirac notation, adjust bracket sizes, and more.

I modified _includes/js-selector.html to add support for the physics package. I also added an example using the physics package in _posts/2019-08-08-text-and-typography.md.

More extensions of MathJax can be found here

Additional context

Fixed #622

[cotes2020]

Thanks @huanyushi, this PR just loads a certain extension (physics). I think it would be possible to move the mathjax configuration to a js config file so that extensions can be configured on demand.

[huanyushi]

I think it would be possible to move the mathjax configuration to a js config file so that extensions can be configured on demand.

@cotes2020 Of course, I have updated this PR. I moved the MathJax configuration from _includes/js-selector.html to a new file _includes/mathjax-support.html.

I removed the physics extension that I had added. Users can now add any configurations they need to _includes/mathjax-support.html.

[cotes2020]

See #1670