Remove WithStringer and enhance anonymous key debugging

[mpyw]

Preparation for v0.2.0 release. Removes redundant functionality and enhances anonymous key debugging experience.

Breaking Changes

Removed	Reason
WithStringer option	Redundant (named keys are sufficient, anonymous keys now auto-include debug info)
StringerFunc type	No longer needed after WithStringer removal

Enhancements

Automatic Debug Info for Anonymous Keys

Keys created without a name now automatically include their definition location (file path and line number).

// Before (v0.1.x)
var Flag = feature.NewBool()
fmt.Println(Flag) // anonymous@0x14000010098

// After (v0.2.0)
var Flag = feature.NewBool()
fmt.Println(Flag) // anonymous(/path/to/file.go:42)@0x14000010098

This allows you to quickly identify where a key was defined, even if you forgot to name it.

Changes

• Use runtime.Caller to capture call site information
• Add appendCallerDepthIncr for accurate stack depth tracking
• Update doc comments and README with new format documentation
• Extract anonymous key tests to anonymous_test.go (ensures stable line numbers)

Test Plan

• go test -v -race all pass
• go vet ./... no issues
• All Example tests work correctly"

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