Add stylesheet() fn and add arguments to show/head/tail

[zkamvar]

Description

This adds a new function called stylesheet() that just returns the path to the internal {tinkr} stylesheet. Moreover, it adds stylesheet_path options to the show(), head(), and tail() methods of the yarn object, which allows other packages to inherit the yarn class and modify these with their own stylesheets

Related Issue

carpentries/pegboard#35

[maelle]

This sounds useful, thank you!