feat: allow loading PdfWrap from custom dynamic library

[wxxedu]

Rationale:

There has been some problems for me loading the dynamic library on Android platform. After checking, I realized that it was because PdfWrap loads the dynamic library at path '/libpdfium.so', but it should be libpdfium.so. This is because:

ibPath = path.join(Directory.current.path, '...');

in the constructor.

While there are many ways of fixing this, I think that the simplest one of them is to just allow the user to specify the dynamic library and when the user does this, it overrides the logic of the dynamic library in the code.