Switch type argument syntax (#1940)

This changes type argument syntax to "standard" style, and away from turbofish.

The motivation here is that turbofish-style type arguments are hard to read, and developers who are not already deeply knowledgeable about Pkl might struggle to understand such code.
In most languages, the grammar for "method call with type args" don't introduce any additional symbols between the identifier and the type argument list (e.g. C++, TypeScript, C#, Scala, Go).

stdlib/syntax.pkl

@@ -305,7 +305,6 @@ typealias TerminalNodeType =
     | "assign"
     | "at"
     | "colon"
-    | "dcolon"
     | "comma"
     | "dot"
     | "gt"