Update to zig 0.14.x plus some fixes

[renerocksai]

Hey, thank you for making this avaliable on its own and your subcomannd enhancements! I love TB flags and, by extension, this project 😄!

I tried to use zli in a current zig project and thought my changes might help:

• I updated the code to work with zig 0.14.0 (std lowercase changes).
• Added a build.zig.zon for use with zig's package manager (specified version v0.1.1; change as you wish).
• For runtime errors (like parsing -n 3), I replaced asserts with fatals and provided meaningful error messages.
• I updated the installation instructions in the README to use the zig package manager.
  • Word of warning: I assumed you'd release v0.1.1 in the REAME.
  • If you choose a different version, just update the string after the # in the URL:
  • zig fetch --save "git+https://github.com/TSxo/zil#v0.1.1"
• I also added support for positionals with default values, including optionals with null as default.