feat: reading multiple pdf files with a single PDFParser object

[nicolabaesso]

Elements changed:

1. Added new test case in a separate file
2. Added the two example PDFs
3. Add the reset of the pages array when the data variable is null

I've added this elements because in my corporate job we are using this library, and recreating everytime the PDFParser object is not something I'm a fan of.
Other test cases are not failing, so no regressions.

[modesty]

thanks for adding more tests. A few thoughts on making the instance of PDFJSClass reusable:

1. pdfparser instance (or the client that instantiates PDFJSClass) needs to be reset/reusable whenever PDFParser is created. (line 107 of pdfparser.js)
2. lib/pdf.js: setting this.pages=[] is not sufficient to dispose the object, pdfDocument and rawTextContents needs reset too. Recomment to call existing destroy method.

[nicolabaesso]

Hi @modesty,
thank you for your review. As suggested, I removed the this.pages=[] line and instead called the already available destroy() function.
Also I've added a function to reset the PDFJS object, is this what you were mentioning? Otherwise let me know.
I've also removed the if in line 120 of pdfparser.js, it was a leftover of one test I was doing to understand the code.