fix: support decorators on abstract classes

[ryanatkn]

Decorators on abstract classes currently fail to parse when not exported:

@decorator
abstract class ExampleClass {}

SyntaxError: Leading decorators must be attached to a class declaration.

Already works:

@decorator
export abstract class ExampleClass {}

This fixes it, adding the test case and a second for when export is included, which already works.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0fc2305

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/acorn-typescript	Patch

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