feat: allow use of indeterminate page ranges

[varevarao]

• Puppeteer, and other PDF generation options allow use of a indeterminate page range format (3-, implying page 3 to the end of the file)
• This would have been possible for consumers by only directly obtaining a page count using the pdf doc instance
• Updated and added tests for the new functionality

Example:

// Add only the first page of Doc A
await merger.add(path.join(FIXTURES_DIR, 'Testfile_A.pdf'), '1')
// Add page 2 onwards from Doc B
await merger.add(path.join(FIXTURES_DIR, 'Testfile_AB.pdf'), '2-')

OR

// Add all pages until the 5th page of Doc A
await merger.add(path.join(FIXTURES_DIR, 'Testfile_A.pdf'), '-5')
// Add page 2 onwards from Doc B
await merger.add(path.join(FIXTURES_DIR, 'Testfile_B.pdf'), '2-')

[Nico-Mugi]

That's a nice feature that I need for my current project, would be nice if it's merged to the repo.
Tested your modification to the code and works like a charm ! Thank you