createDebugConfiguration is silently failing returning an empty
list of debug configurations.

We are incorrectly registering a single debug provider that provides
`cppdbg` and `cppvsdbg` configurations using the `cppdbg` type.

Split the cppdb and cppvsdbg debug providers into 2 different providers
and register them based on the current operating system following the
same approach as the `ms-vscode.cpptools` extension.

Also simplify the debug providers into a single MesonDebugProvider
class.

Providing debug configurations if the debugger extension is not
installed has 2 problems:
  * The debug configuration is listed in a menu item named "undefined"
  * The launch configuration will not work because the debugger does not
    exists

We shouldn't infer what debugger to use based on the compiler
command. Instead we use the defaults based on the OS:
https://code.visualstudio.com/docs/cpp/launch-json-reference#_mimode