fix: initialize default Usage function on NewFlagSet and Init

[AdamMagued]

Problem

In the Go standard library flag package, calling flag.NewFlagSet(name, errorHandling) initializes f.Usage to point to a default usage printer.
In spf13/pflag, NewFlagSet and Init leave f.Usage uninitialized (nil). Consequently, if user code or third-party libraries create a FlagSet and invoke f.Usage(), the program panics with a nil pointer dereference.

Root Cause

NewFlagSet and FlagSet.Init did not assign a default function to f.Usage. While internal methods like f.usage() had a fallback when f.Usage was nil, direct invocations of the exported f.Usage field resulted in a nil function call panic.

Solution

1. In flag.go, update NewFlagSet and Init to initialize f.Usage = func() { defaultUsage(f) }, matching Go standard library flag behavior.
2. In flag_test.go, add TestNewFlagSetDefaultUsage and TestFlagSetInitDefaultUsage to verify that newly constructed flag sets have a non-nil Usage function that prints the expected usage output without panicking.

Verification

• Ran full test suite in container sandbox (go test -v ./...); all tests pass.
• Verified specific unit tests for both NewFlagSet and FlagSet.Init.

Fixes #253

[CLAassistant]

All committers have signed the CLA.

[AdamMagued]

Updated in 9c50e03: added docstrings for FlagSet.Usage, NewFlagSet, and FlagSet.Init documenting the default Usage function initialization as requested in review.