Disambiguate more towards new given syntax, show solution for scala#1…

…5840

Faced with

    given C[T]:
      ...

(with a new line after `:`) we now classify this as new given syntax, and assume ... is a template body.
If one wants to use old syntax, one can still write

    given C[T]
      : ImplementedType ...

# Conflicts:
#	compiler/src/dotty/tools/dotc/parsing/Parsers.scala

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -965,7 +965,6 @@ object Parsers {
       lookahead.isColon
       && {
         !in.featureEnabled(Feature.modularity)
-        || paramsSeen
         || { // with modularity language import, a `:` at EOL after an identifier represents a single identifier given
              // Example:
              //    given C:

tests/run/i15840.scala

@@ -0,0 +1,27 @@
+//> using options -language:experimental.modularity -source future
+
+trait Nat:
+  type N <: Nat
+
+class _0 extends Nat:
+  type N = _0
+
+class NatOps[N <: Nat](tracked val n: N):
+  def toInt(using toIntN: ToInt[n.N]): Int = toIntN()
+
+// works
+def toInt[N <: Nat](n: N)(using toIntN: ToInt[n.N]) = toIntN()
+
+sealed abstract class ToInt[N <: Nat]:
+  def apply(): Int
+
+object ToInt:
+  given ToInt[_0] {
+    def apply() = 0
+  }
+
+@main def Test() =
+  assert(toInt(new _0) == 0)
+  assert(NatOps[_0](new _0).toInt == 0)
+  assert:
+    NatOps(new _0).toInt == 0 // did not work