fix(#18265): crash on extension method without body

This commit fixes a bug that causes Scala compiler to crash due to position error
when an extension method followed by a newline has neither type annotation nor `= <expr>`.

This commit is based on #18445.

Co-Authored-By: @hamzaremmal

Author: i10416

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

@@ -2577,6 +2577,8 @@ object Parsers {
       parents match {
         case parent :: Nil if !in.isNestedStart =>
           reposition(if (parent.isType) ensureApplied(wrapNew(parent)) else parent)
+        case tkn if in.token == INDENT =>
+          New(templateBodyOpt(emptyConstructor, parents, Nil))
         case _ =>
           New(reposition(templateBodyOpt(emptyConstructor, parents, Nil)))
       }

tests/neg/i18265.check

@@ -0,0 +1,6 @@
+-- [E019] Syntax Error: tests/neg/i18265.scala:5:13 --------------------------------------------------------------------
+5 |    def twice // error
+  |             ^
+  |             Missing return type
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i18265.scala

@@ -0,0 +1,5 @@
+trait Foo
+
+val foo = new Foo:
+  extension (s: String)
+    def twice // error