fix error message on setter with wrong type

tests/neg/i20338a.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i20338a.scala:10:15 -----------------------------------------------------------
+10 |  test.field = "hello" // error
+   |               ^^^^^^^
+   |               Found:    ("hello" : String)
+   |               Required: Int
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i20338a.scala

@@ -0,0 +1,10 @@
+object types:
+  opaque type Struct = Int
+  val test: Struct = 25
+  extension (s: Struct) 
+    def field: Int = s
+    def field_=(other: Int) = ()
+
+@main def hello = 
+  import types.*
+  test.field = "hello" // error

tests/neg/i20338b.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i20338.scala:10:8 -------------------------------------------------------------
+10 |  f.x = 42 // error
+   |        ^^
+   |        Found:    (42 : Int)
+   |        Required: String
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i20338b.scala

@@ -0,0 +1,10 @@
+class Foo(_x: Int)
+
+extension (s: Foo)
+  def x_=(x: String): Unit = ()
+  def x: Int = ???
+
+@main
+def Test =
+  val f = Foo(42)
+  f.x = 42 // error

tests/neg/i20338c.check

@@ -0,0 +1,6 @@
+-- [E052] Type Error: tests/neg/i20338c.scala:9:6 ----------------------------------------------------------------------
+9 |  f.x = 42 // error
+  |  ^^^^^^^^
+  |  Reassignment to val x
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i20338c.scala

@@ -0,0 +1,9 @@
+class Foo(val x: Int)
+
+extension (s: Foo)
+  def x: Int = 43
+
+@main
+def Test =
+  val f = Foo(42)
+  f.x = 42 // error