Add support for standardize() on objects of class fixest

[etiennebacher]

Fixes #664

[mattansb]

(Cool!)

[etiennebacher]

Ping @strengejacke, I'm not so familiar with standardizing directly the regression results so I'd like a double check on this, just in case there are some gotchas I'm not aware of.

[mattansb]

The regression results are standardized in {parameters}. Here we refit the model with standardized data.

So generally if these two give the same result, it works.

mod1 <- model_fitting_function(..., data = d)
mod1_z <- standardize(mod1)

mod2_z <- model_fitting_function(..., data = standardize(d))

(Depending link and transformations, might not be exactly the same)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.75%. Comparing base (9a624b1) to head (54ceae9).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #665      +/-   ##
==========================================
+ Coverage   91.73%   91.75%   +0.02%     
==========================================
  Files          76       76              
  Lines        6410     7664    +1254     
==========================================
+ Hits         5880     7032    +1152     
- Misses        530      632     +102     

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[etiennebacher]

Ok I'll go ahead with this, thanks for the review @mattansb