Reject abstract members in non-abstract classes (#1647)

Fixes #1614.

## Context

A non-abstract `class` (or `module`) was allowed to declare `abstract`
properties and methods.
Because such an enclosing type is instantiable, an `abstract` member
there can never be guaranteed
an implementation — so the contradiction surfaced only as a runtime
error when the member was
accessed (`Cannot invoke abstract method`), or not at all.

This makes it a compile-time error to declare an `abstract` member
unless its enclosing class or
module is also `abstract`. This is consistent with how Pkl already
rejects instantiating an abstract
class, and mirrors how Java and Kotlin treat abstract members.

## Before

```pkl
class Foo {
  abstract bar: Int
}
res = new Foo { bar = 5 }   // evaluated successfully (should fail)
```

```pkl
class Foo {
  abstract function bar(): Int
}
res = new Foo {}            // evaluated successfully; res.bar() failed only at runtime
```

## After

```
–– Pkl Error ––
Cannot define an abstract member in a non-abstract class.

2 | abstract bar: Int
    ^^^^^^^^
at Foo

A member can only be `abstract` if its enclosing class is also `abstract`.
```

## Implementation

- `AstBuilder` now validates, while building the AST, that a
non-abstract class/module declares no
`abstract` members. The check runs in both `visitClass` and
`visitModule`, and the error points at
  the `abstract` keyword.
- Adds the `abstractMemberInNonAbstractClass` error message.

## Scope: classes and modules

The issue describes classes; I applied the same rule to modules as well,
since a module is a class
in Pkl and a non-abstract module is likewise directly evaluatable. Happy
to narrow this to classes
only if you'd prefer — it's a one-line change either way.

The `moduleMethodModifiers` pkl-doc test fixture declared an abstract
method at non-abstract module
level (relying on the old behavior); it's updated to an `abstract
module`, and its expected
documentation output is regenerated.

## Tests

- New `LanguageSnippetTests` error cases: abstract property in a class,
abstract method in a class,
  and abstract member in a module.
- `./gradlew build` passes (`pkl-core` and `pkl-doc` included).

---------

Co-authored-by: Vinayak <vinayak@vama.app>
Co-authored-by: Daniel Chao <daniel.h.chao@gmail.com>

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>