emHeightDescent and emHeightAscent return value that should returns from fontBoundingBoxAscent and fontBoundingBoxDescent

[musou1500]

According to specification, fontBoundingBoxAscent returns following.

The distance from the horizontal line indicated by the textBaseline attribute to the ascent metric of the first available font.

ascent metric is generally takes from font metrics.
(see https://drafts.csswg.org/css-inline/#ascent-descent)

node-canvas seems implemented it in emHeightAscent.
https://github.com/Automattic/node-canvas/blob/master/src/CanvasRenderingContext2d.cc#L2778-L2779

about em-square, ascent and descent, see https://learn.microsoft.com/en-us/typography/opentype/spec/ttch01#funits-and-the-em-square .

my investigation result on Firefox.

related #1940

Your Environment

• Version of node-canvas (output of npm list canvas or yarn list canvas):
• Environment (e.g. node 20.9.0 on macOS 14.1.1):