Breaking: Completely rework how positional arguments are handled

[FollowTheProcess]

Summary

Warning

This is a large breaking change

I've completed re-worked how positional arguments work, instead of getting the raw []string passed to the Run method, I've introduced a mechanism
for declaring type-safe structs describing the positional arguments, which are then inferred using generics and parsed similar to how flags already
worked.

As a result of this:

• I've dropped the cli.Allow option and all the argument validators as this new mechanism is much better and allows you to define exactly what
  arguments you want and what types they are
• The signature of the function provided to cli.Run has changed from func(cmd *cli.Command, args []string) error to func(cmd *cli.Command) error
• Raw positional arguments can still be obtained using cmd.Args()

See the updated README and the updated examples for how to change your code to match the new behaviour

Note

This is not yet complete, I still want to work out how slice types work in this new system but this is good enough for now

[codecov]

Codecov Report

❌ Patch coverage is 78.75000% with 85 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.06%. Comparing base (14524a0) to head (770905d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/arg/arg.go	76.41%	63 Missing and 12 partials ⚠️
command.go	79.48%	5 Missing and 3 partials ⚠️
option.go	94.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
- Coverage   91.50%   87.06%   -4.44%     
==========================================
  Files           5        5              
  Lines        1260     1477     +217     
==========================================
+ Hits         1153     1286     +133     
- Misses         61      129      +68     
- Partials       46       62      +16     

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