Add step filter configuration

[Jamie Thompson]

add configuration compatible with vscode launch.json, that allows fine grained control over the step-into functionality of the debugger (and possibly also the stack trace filtering)

some ideas:

• skip classes in (package foo/static path bar/library x)
• only pause on classes in (package foo/static path bar/library x)
• only pause on classes in x, y, z subproject
• skip $JDK, $SCALA_LIB, etc (special tokens for various "Standard" libraries)