Add function to dynamically determine package version for PKGBUILD

[timwehrle]

Description:
This pull request resolves issue #176 by enhancing the PKGBUILD file to dynamically set the pkgver based on Git tags.

Changes:

1. PKGBUILD:
   • Implemented a pkgver() function that dynamically retrieves the latest Git tag using git describe --tags.
   • The function uses sed 's/^v//;s/-.*//' to format the version string, ensuring it adheres to the pkgver format by:
     • Stripping the leading v from the tag (e.g., v1.7.1 becomes 1.7.1).
     • Removing any additional commit metadata (e.g., -3-ge168780).

[kehoecj]

@timwehrle Can you show me some output from testing this on Arch?