Add devcontainer

[thumperward]

This PR adds a VSCode devcontainer configuration which enables the package to be built in a Docker container with all dependencies isolated from the host system. This removes the need to install or configure Java, Maven, Launch4j etc locally. Just open the repo in VSCode, choose to re-open in the devcontainer, and run mvn package as normal.

The diff has been minimised to only include the devcontainer config and that automatically generated by Maven, with the exception of one code change: a cast which fixes Swing issues in recent versions of Java.

The two README files in the root directory have been merged and tidied.

This is an initial pull to add the vscode logic; if acceptable then there are follow-ups to clean up code, add pre-commit hooks etc.