Modernize JS build scripts (#19824)

- Remove __dirname, use file URLs instead
- Upgrade fabric dependency
- Use fs/promises syntax, this breaks node 12 but we require 14 already

The change in public/img/favicon.svg is not caused by the fabric
upgrade, but it seems it was not properly generated when introduced.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Makefile

@@ -761,7 +761,7 @@ generate-gitignore:
 
 .PHONY: generate-images
 generate-images: | node_modules
-	npm install --no-save --no-package-lock fabric@4 imagemin-zopfli@7
+	npm install --no-save --no-package-lock fabric@5 imagemin-zopfli@7
 	node build/generate-images.js $(TAGS)
 
 .PHONY: generate-manpage