Allow 3rd-party extensions to contribute in-memory launch config

Currently, vscode-java-debug searches the .vscode/launch.json file for a launch config matching the classname+project name.

https://github.com/microsoft/vscode-java-debug/blob/415ae57296c10e9ffd2204ab66e22cb9d929259c/src/extension.ts#L450-L459

If none is found, a default config is built in memory:

https://github.com/microsoft/vscode-java-debug/blob/415ae57296c10e9ffd2204ab66e22cb9d929259c/src/extension.ts#L427-L433

I'd like the JBang extension to provide a custom in-memory config, where we can dynamically set the add-opens args for instance, to be able to keep in synch' with the JBang directives in the java file, rather than having to create/update .vscode/launch.json at some point.

Would be nice if vscode-java-debug allowed 3rd-party extension to contribue launch config participants, that, if they match, would then provide a custom in-memory config, if none was found on disk.

See the video at the 4'37" mark and https://twitter.com/fbricon/status/1698735665259626996 for context

cc @maxandersen

Start with the launch-config lookup and fallback construction in src/extension.ts at lines 427-433 and 450-459. Trace how the default in-memory configuration is selected, then determine how a third-party participant such as JBang could contribute a matching configuration. Done means an extension can provide a custom in-memory config when no on-disk match exists, including dynamically derived add-opens arguments.