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-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`.