Revert "Revert "Reject abstract members in non-abstract classes" (#1688)" (#1780)

This reverts commit 1bf00b84ea.

Co-authored-by: Vinayak <vinayak@vama.app>

pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractClass.pkl

@@ -0,0 +1,5 @@
+class Foo {
+  abstract bar: Int
+}
+
+res = new Foo { bar = 5 }

pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMemberInNonAbstractModule.pkl

@@ -0,0 +1 @@
+abstract foo: Int

pkl-core/src/test/files/LanguageSnippetTests/input/errors/abstractMethodInNonAbstractClass.pkl

@@ -0,0 +1,5 @@
+class Foo {
+  abstract function bar(): Int
+}
+
+res = new Foo {}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractClass.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract bar: Int
+    ^^^^^^^^
+at abstractMemberInNonAbstractClass#Foo (file:///$snippetsDir/input/errors/abstractMemberInNonAbstractClass.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.

pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMemberInNonAbstractModule.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract foo: Int
+    ^^^^^^^^
+at abstractMemberInNonAbstractModule (file:///$snippetsDir/input/errors/abstractMemberInNonAbstractModule.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.

pkl-core/src/test/files/LanguageSnippetTests/output/errors/abstractMethodInNonAbstractClass.err

@@ -0,0 +1,8 @@
+–– Pkl Error ––
+Cannot define an abstract member in a non-abstract class.
+
+x | abstract function bar(): Int
+    ^^^^^^^^
+at abstractMethodInNonAbstractClass#Foo (file:///$snippetsDir/input/errors/abstractMethodInNonAbstractClass.pkl)
+
+A member can only be `abstract` if its enclosing class is also `abstract`.