Add Unique() filter method

[HakeemTheEmperor]

Resolves #247

Changes Introduced

Adds a Unique() method to Pipe (using FilterScan) that collapses consecutive duplicate lines while maintaining chronological order and real-time streaming capability.

Testing

• Added TestUnique in filters_test.go covering consecutive duplicates, non-consecutive duplicates, unchanged unique streams, and empty pipes.
• Verified test coverage is 100%.

Example

script.Echo("a\na\nb\na\n").Unique().Stdout()
// Output:
// a
// b
// a