fix: remove concrete subclasses of ColumnType

packages/safe-ds-lang/src/resources/builtins/safeds/data/tabular/typing/column_type.sdsstub

@@ -0,0 +1,26 @@
+package safeds.data.tabular.typing
+
+/**
+ * Abstract base class for column types.
+ *
+ * @param isNullable Whether the columntype is nullable.
+ */
+class ColumnType {
+    /**
+     * Return whether the given column type is nullable.
+     *
+     * @result result1 True if the column is nullable.
+     */
+    @Pure
+    @PythonName("is_nullable")
+    fun isNullable() -> result1: Boolean
+
+    /**
+     * Return whether the given column type is numeric.
+     *
+     * @result result1 True if the column is numeric.
+     */
+    @Pure
+    @PythonName("is_numeric")
+    fun isNumeric() -> result1: Boolean
+}

packages/safe-ds-lang/src/resources/builtins/safeds/data/tabular/typing/_schema/_schema.sdsstub → packages/safe-ds-lang/src/resources/builtins/safeds/data/tabular/typing/schema.sdsstub

@@ -7,9 +7,7 @@ from safeds.data.tabular.typing import ColumnType
  *
  * @param schema Map from column names to data types.
  */
-class Schema(
-    `schema`: Map<String, ColumnType>
-) {
+class Schema {
     /**
      * Return a list of all column names saved in this schema.
      */