Add syntax highlighting to README

[coliff]

PR Checklist

I made this PR using GitHub Web UI so I was unable to run tests. As this is a README-only change hopefully that's ok!

• [x] read and followed the CONTRIBUTING.md guide.
• [ ] built and tested the changes locally.
• [ ] added/updated tests.
• [ ] added/updated API documentation.
• [ ] added/updated demos.

[coliff]

This makes the README a little nicer to view. Preview using GitHub Rich-diff UI.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.91%. Comparing base (5c52605) to head (657d70c).

Additional details and impacted files

@@               Coverage Diff               @@
##           development    #6603      +/-   ##
===============================================
+ Coverage        76.85%   76.91%   +0.05%     
===============================================
  Files              315      315              
  Lines            10668    10668              
  Branches          2848     2848              
===============================================
+ Hits              8199     8205       +6     
+ Misses            2468     2462       -6     
  Partials             1        1              

see 6 files with indirect coverage changes

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