Make it possible to run the debugger in the dotty repo

[adpi2]

BSP is disabled in the bootstraped projects of the compiler for better performance of the IDEs.

But if we bump the TASTy version, we cannot run the un-boostrapped compiler without the bootstrapped library: the compiler needs to read the library with the correct version of TASTy.

One possible solution is to override the debug task in the un-bootstrapped compiler to run the bootstrapped compiler.

[adpi2]

One possible solution is to override the debug task in the un-bootstrapped compiler to run the bootstrapped compiler.

Tried that but it did not work because Metals or the debugger could not configure any breakpoint.

Alternatively I managed to make it work by specifying the -classpath explicitly with the scala3-library-bootstrapped and the scala-library (Scala 2.13).

[adpi2]

Started writing a tutorial in the Scala 3 contributing guide.

Quentin from LARA and Tim from LAMP are testing it and reporting the issues.
We found some minor bugs to fix: #387

[adpi2]

Closed by scala/docs.scala-lang#2757