Normalize :: syntax

[helixbass]

@GeoffreyBooth a couple Prettier-related changes to try and make generation of :: constructs safer:

• Currently, ., ?. and :: are LINE_CONTINUERs but ?:: is not. So add it so that line-breaking before any kind of Accessor (other than a "computed" one, ie an Index eg ['a']) should be safe
• Currently, there is "standalone" :: (ie just referencing the .prototype object) as well as :: followed by a Property. ?:: has no standalone version in the grammar. So add it, since currently in Prettier I'm auto-transforming any .prototype access into ::, so if the original source were eg a?.prototype it'd generate a?::, which currently won't compile. If it ends up making sense to only generate :: when explicit :: was in the original source, that should just be a question of eg adding a prototypeShorthand: yes flag to the Access in the grammar, but there's no harm in allowing standalone ?:: regardless

[GeoffreyBooth]

@zdenko or @vendethiel ?

[connec]

Seems like a well-scoped change. Only one comment:

• Would it be worth adding a test explicitly for soaked :: (e.g. test the soaking behaviour, rather than the line continuation) in soaks.coffee?

[helixbass]

@connec looks like there were existing checks that ?:: behaves as expected in test/operators.coffee, so i expanded that test to test that ?:: behaves the same way when continuing a line