build(core): kill 8080 port before serving vue files

[ivikash]

Problem

AWS Toolkits and Amazon Q extension expects the web view to be served at 8080 port. When there is a existing process, vue process starts at 8081 and so on ports. When this happens, webview doesn't always serve the latest code hampering developer productivity.

Solution

• Kill the process and free up 8080 port before starting serveVueCore process. This ensures webview is always served on port 8080.
• kill-port in cross platform.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.