Fix gulp watch task (#6117)
Fix gulp watch task (#6117)

- revise watch & task syntax for Gulp v4 requirements
- ensure only one watcher gets created for file type paths
takeflight/wagtail,rsalmaso/wagtail,torchbox/wagtail,thenewguy/wagtail,thenewguy/wagtail,FlipperPA/wagtail,mixxorz/wagtail,jnns/wagtail,kaedroho/wagtail,zerolab/wagtail,rsalmaso/wagtail,jnns/wagtail,wagtail/wagtail,kaedroho/wagtail,jnns/wagtail,takeflight/wagtail,mixxorz/wagtail,thenewguy/wagtail,zerolab/wagtail,zerolab/wagtail,wagtail/wagtail,gasman/wagtail,takeflight/wagtail,gasman/wagtail,torchbox/wagtail,torchbox/wagtail,mixxorz/wagtail,mixxorz/wagtail,zerolab/wagtail,rsalmaso/wagtail,thenewguy/wagtail,rsalmaso/wagtail,wagtail/wagtail,gasman/wagtail,wagtail/wagtail,FlipperPA/wagtail,gasman/wagtail,kaedroho/wagtail,takeflight/wagtail,FlipperPA/wagtail,gasman/wagtail,wagtail/wagtail,kaedroho/wagtail,rsalmaso/wagtail,zerolab/wagtail,jnns/wagtail,thenewguy/wagtail,torchbox/wagtail,kaedroho/wagtail,mixxorz/wagtail,FlipperPA/wagtail