Add prettier to Maven/Gradle CSS extensions

[fvgh]

For Eclipse WTP formatters Maven/Gradle extensions have been added, providing default file selection and header delimiter.
Prettier has been introduced with #283 as a generic formatter step.
It should be added to:

• CSS extension (Eclipse-WTP CSS integration #311 )

NOTE: Issue is incomplete. More extensions fill follow soon.

[fvgh]

Issue is obsolete due to agreed changes on WTP integration.