Add Visual Studio Code workspace generation support (for native code)

Visual Studio Code projects (workspaces) use the compile_commands.json file to provide source code indexing of native code. Since the location of this file is build specific, it would be useful to allow generation of matching project files for a certain build configuration.

Additionally, there are several alternative source code indexers available for VS Code, which all have their specific configuration for finding the compile_commands.json. The project files can optionally include configuration for indexers that are known to work with the JDK native source.

Currently proposed/implemented new build targets are:

* vscode-project (uses the default "cpptools" indexer)
* vscode-project-clangd
* vscode-project-rtags