`Activity diagrams`: Enhance fork node connectors

[minrows]

Checklist

• [x] I added multiple screenshots/screencasts of my UI changes

Motivation and Context

This PR enforces the connector of the Fork node to be straight while connecting to other elements.

Steps for Testing

1. Clone the repo
2. Checkout to enhancement/fork-node-connectors branch
3. Execute commands: yarn install, yarn build followed by yarn start
4. Try creating a new Sequence diagram as illustrated in Screenshot below
5. Observe that all the connectors connecting to/from fork node are straight, as illustrated in after section of the Screenshot

Screenshots

Before:

After:

Additional Info

Released in alpha version v2.11.0-alpha.0