Update configuration and fix deprecation warnings related to charlists

[m0to]

• Update config.exs to current specification using import Config
• Update single quoted charlists to use the character sigil
• Improve pid validation in xlsx_file.ex
• Add explicit conversion of paths to charlist in unzip.ex
• ran mix format on all files

[jamesvl]

I'm hopeful the maintainer here will pick this up and keep the package up to date.

In the mean time, I prefer to have less compiler warnings and bugs vs. more, so I've forked and then merged in your MR to the main branch of https://github.com/Semsee/xlsxir

I've also run mix deps.update --all and verified unit tests are still passing. Since my organization is using this package I'm happy to maintain things there for the foreseeable future.