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

This reverts commit 1bf00b84ea.

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

pkl-doc/src/test/files/DocGeneratorTest/input/com.package1/moduleMethodModifiers.pkl

@@ -1,5 +1,5 @@
 /// Module methods with different modifiers.
-module com.package1.moduleMethodModifiers
+abstract module com.package1.moduleMethodModifiers
 
 /// Method with `abstract` modifier.
 abstract function method1(arg: String): Boolean

pkl-doc/src/test/files/DocGeneratorTest/output/run-1/com.package1/1.2.3/moduleMethodModifiers/index.html

@@ -32,7 +32,7 @@
       </ul>
       <div id="_overview" class="anchor"> </div>
       <div id="_declaration" class="member">
-        <div class="member-signature">module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
         <div class="doc-comment"><p>Module methods with different modifiers.</p></div>
         <dl class="member-info">
           <dt class="">Module URI:</dt>

pkl-doc/src/test/files/DocGeneratorTest/output/run-2/com.package1/1.2.3/moduleMethodModifiers/index.html

@@ -32,7 +32,7 @@
       </ul>
       <div id="_overview" class="anchor"> </div>
       <div id="_declaration" class="member">
-        <div class="member-signature">module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
         <div class="doc-comment"><p>Module methods with different modifiers.</p></div>
         <dl class="member-info">
           <dt class="">Module URI:</dt>