Remove `as` syntax

[nicolasstucki]

All uses of as as a keyword have been removed. We need to sync with the new syntax.

• It was used in patterns
• It was used in given definitions (see http://dotty.epfl.ch/docs/reference/contextual/givens.html)

[nicolasstucki]

It looks like we just need to remove the following pattern Scala.tmLanguage.ts#L569-L574. We should also update any code that used the old syntax.

[nicolasstucki]

See #163