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

The changes made in this commit are good, but we're going to kick this out
to the next release.
This is because:

1. There's a couple more issues around the `abstract` modifier that is not
implemented yet, and need design considerations
2. These are breaking changes, and we want to minimize the amount of breakages
for users.
3. The main branch is still the develop branch for Pkl 0.32

We will apply a re-revert of this commit after Pkl 0.32 is released.

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

@@ -1,5 +1,5 @@
 /// Module methods with different modifiers.
-abstract module com.package1.moduleMethodModifiers
+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">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">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">abstract module <span class="name-decl">com.package1.moduleMethodModifiers</span></div>
+        <div class="member-signature">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>