Create mutate_feature for mutating a feature node exclusively

[MilesCranmer]

This also make template expressions have smarter mutations wrt features. They will only generate feature nodes that are valid. This is much better than the current approach which is basically generate mutations until we find a valid expression.

[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (1c5920d) to head (348f127).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/TemplateExpression.jl	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
+ Coverage   94.76%   94.77%   +0.01%     
==========================================
  Files          42       42              
  Lines        3725     3753      +28     
==========================================
+ Hits         3530     3557      +27     
- Misses        195      196       +1     

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