[Webpack Issue] The path resolve problem in build stage

**Describe the bug** This bug will trigger the following error at deployment time ``` [ERR_INVALID_ARG_TYPE]: The "paths[0]" argument must be of type string. Received type number (91100) ``` **To Reproduce** Steps to reproduce the behavior: 1. Use webpack to build the project 2. Invoke the `pdf2md` function 3. See the error at the console **Expected behavior** The path dependence seems will break when using `webpack` to build the production, which the same code work smoothly at development mode **Screenshots**  **Desktop (please complete the following information):** - OS: Window - VSCode - Version 1.93.0 **Additional context** Yes, seems like the path in the dependence really mass up, I have tried to mark it as `externals`, but sadly it does not work for me. [Related Issue: Support Webpack, don't use require.resolve() #76](https://github.com/opengovsg/pdf2md/issues/76)