Adapt syntax highlighting to stabilized Dotty syntax

[odersky]

The Syntax from Dotty 0.22 on uses several new soft keywords that need to be properly syntax highlighed. In particular:

• extension Ident? on (both extension and on should be highlighted)
• using after opening "("
• as following a given signature. A given signature consists of given, an optional identifier, and optional type parameters [...] and context parameters (using ...).
• open as a soft modifier

[nicolasstucki]

Replaced with #91, #86, #87, #88