Add CI job to check the MSRV

[FreezyLemon]

The job only checks a default x86 build and a minimal wasm32 build. That should be enough to find most problems before starting the big build matrices.

This addresses the MSRV part of #3363. Tests are still needed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.54%. Comparing base (3c3a26f) to head (fe8e83b).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3364      +/-   ##
==========================================
+ Coverage   88.52%   88.54%   +0.01%     
==========================================
  Files          89       89              
  Lines       28323    28355      +32     
==========================================
+ Hits        25074    25107      +33     
+ Misses       3249     3248       -1     

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