… packages

* Split the helpers into separate packages `@signpdf/placeholder-pdfkit010` and `@signpdf/placeholder-plain`;
* placeholder helpers now require `contactInfo`, `location` and `name` which used to be optional;
* Introduced more types declarations through TS and JSDoc;
* Introduced examples in `packages/examples`;

Introducing @signpdf/signer-p12 and making signpdf's sign() method async

Co-authored-by: Daniel Hensby <[email protected]>