feat: new library UML Sequence Diagram Symbols

[excalibot]

A library of prebuilt symbols for UML Sequence Diagrams including: • Object Lifelines (1-6) • Activation box • Messages (sync, async, and return) • Self-messages • Alternative / Parallel Fragments • Loop / Optional / Reference / Break fragments     •   Termination Symbol

Submitted by @djankies

Items: Opt/Loop/Ref/Break, Activation Box, Termination, Self Message, Response Message, Async Message, Sync Message, Object Lifelines (6), Object Lifelines (5), Object Lifelines (4), Object Lifelines (3), Object Lifeline (1), Alt/ Par

Installation link