Restrict set of tokens after which a colon argument is allowed

Drop strings and quoted idents. That leaves:

 - alphanumeric identifiers, backticked identifiers, this, super, `)`, `]`

This choice was made to keep the extension minimal. Quite a few files in the
intent project have to be changed since they all use an idiom like

  "Formatting":
    ...

with a colon after a string literal. We can discuss separately whether we want to support this,
but IMO there's no pressing need to do so.

Author: odersky

community-build/community-projects/intent

@@ -283,7 +283,7 @@ object Tokens extends TokensCommon {
 
   final val endMarkerTokens = identifierTokens | BitSet(IF, WHILE, FOR, MATCH, TRY, NEW, THROW, GIVEN, VAL, THIS)
 
-  final val colonEOLPredecessors = BitSet(RPAREN, RBRACKET, BACKQUOTED_IDENT, THIS, SUPER, QUOTEID, STRINGLIT)
+  final val colonEOLPredecessors = BitSet(RPAREN, RBRACKET, BACKQUOTED_IDENT, THIS, SUPER)
 
   final val closingParens = BitSet(RPAREN, RBRACKET, RBRACE)