Implement fixed effects encoding as `formulaic` transformation

[leostimpfle]

Move the encoding of fixed effects to inside the model matrix construction:

pyfixest/pyfixest/estimation/formula/model_matrix.py

Lines 356 to 358 in dec2f1b

	fixed_effects_formula = _encode_fixed_effects(
	fixed_effects=formula.fixed_effects, data=data
	)