Streamline xanchor and yanchor names for center/middle

The 'xanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'left', 'center', 'right']

The 'yanchor' property is an enumeration that may be specified as:
- One of the following enumeration values:
['auto', 'top', 'middle', 'bottom']

This is for plotly 6.3.0 trying to use fig.add_annotations() but it likely applies everywhere where xanchor and yanchor are used.

It does make sense that for the extremes of the bounding box left right top bottom, the names are different, but why is 'center' used for x direction and 'middle' for y direction? This distinction seems artifical to me and I wonder why this was chosen?

I propose to streamline the naming conventions, but now for backwards compatibility to at least accept both center/middle for x/yanchor.

Comece pelo ponto de entrada fig.add_annotations() e localize como os valores de xanchor e yanchor são validados. Verifique se a mesma validação é compartilhada em outros lugares e, em seguida, revise os testes relacionados. A implementação deve incluir, quando apropriado, a aceitação compatível com versões anteriores de center e middle, mantendo o funcionamento dos valores existentes.