Improve results display

[matthieugomez]

Closes #237.

• Show the dependent variable first in the header
• Mark IV models in the title (FixedEffectModel (IV))
• Format the F-test p-value with PValue (<1e-99 instead of 0.000)
• Add a thousands separator to the number of observations
• Drop the Converged row (it was a row filler in the base block); print a warning line under the header instead when demeaning did not converge
• Widen the table when a header label/value pair does not fit in half of it, instead of silently misaligning the row
• Regenerate the README example with the new format

                            FixedEffectModel
=========================================================================
Dependent variable:           Sales   Number of obs:                1,380
dof (model):                      1   dof (residuals):                 45
R²:                           0.803   R² adjusted:                  0.791
F-statistic:                13.3382   P-value:                     0.0007
R² within:                    0.139   Iterations:                       5
=========================================================================
        Estimate  Std. Error    t-stat  Pr(>|t|)   Lower 95%    Upper 95%
─────────────────────────────────────────────────────────────────────────
NDI  -0.00526264  0.00144097  -3.65216    0.0007  -0.0081649  -0.00236038
=========================================================================

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
src/FixedEffectModel.jl	91.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #284   +/-   ##
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  Coverage        ?   97.87%           
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  Files           ?        8           
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