Make shape `ellipse` aware of `/pgf/shape aspect`.

[muzimuzhi]

Brief outline of the proposed feature

Currently it's hard (if not impossible) to set the ratio of x- and y-radii of an ellipse shape.

See tex-sx questions https://tex.stackexchange.com/questions/611345/vertical-ellipse-node and https://tex.stackexchange.com/questions/103557/size-of-tikz-ellipse-node.

Discussion:

• Calculations related to ellipse seems to be complex and inexact.
• The problem of precedence of minimum height/width and aspect keys, as raised in diamond shape: keep aspect when checking against minimum ht/wd #1042, is encountered again.
• Perhaps provide a new shape in order to keep backward compatibility? Considering pgfmanual reads "This shape is an ellipse tightly fitting the text box, ..."