evalengine: Add MID alias

[dbussink]

This is a synonym for SUBSTRING so let's add it as such.

Related Issue(s)

Part of #9647

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (38573f0) 47.64% compared to head (7c9cba1) 47.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15066      +/-   ##
==========================================
+ Coverage   47.64%   47.65%   +0.01%     
==========================================
  Files        1151     1151              
  Lines      239809   239809              
==========================================
+ Hits       114256   114281      +25     
+ Misses     116950   116926      -24     
+ Partials     8603     8602       -1     

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