figure: default-figure-* should be configurable

[sorawee]

The function figure allows a limited adjustment for figure caption styling. But to totally control the styling, we need to be able to configure default-figure-label-text, default-figure-label-sep, default-figure-caption-style, and default-figure-counter-style. These parameters however are only provided in a private module scribble/private/lang-parameters.rkt.

default-figure-counter-style is also a bit odd. It is used inside make-delayed-element (in the function counter-target), so module loading ordering could affect its value.