tex-scalerel{,-doc}: Add version 1.8

The package provides four commands for vertically scaling and
stretching objects. Its primary function is the ability to
scale/stretch and shift one object to conform to the size of a
specified second object. This feature can be useful in both
equations and schematic diagrams. Additionally, the scaling and
stretching commands offer constraints on maximum width and/or
minimum aspect ratio, which are often used to preserve
legibility or for the sake of general appearance.

(markd)