fix: add prototype to errors

[daprieto1]

What?

Add the prototype to each error.

Why?

When you want to validate the instance of the error the result will be alwas false and wrong.

try {
    // The file is encrypted
    return await PDFDocument.load(new Uint8Array(file));
} catch (err) {
    // This condition will be always false
    if (error instanceof EncryptedPDFError) {
    }
}

How?

This is necessary from TypeScript 2.1

Testing?

New Dependencies?

Screenshots

Suggested Reading?

Anything Else?

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.