www/waterfox: Update 6.0.18 → 6.0.19, improve port

Update 6.0.18 → 6.0.19 are trivial - just change version.

Improve port:
 - Make Makefile more look like www/librewolf/Makefile and
   www/firefox-esr/Makefile
 - Remove doubles of USE_GECKO and BUILD_DEPENDS+=devel/wasi-*
 - Fix warnings from portclippy

Changelog:
https://github.com/BrowserWorks/Waterfox/releases/tag/G6.0.19

PR:	281330