[lldb/Interpreter] Make Scripted*Interface base class abstract (#71465)

This patch makes the various Scripted Interface base class abstract by
making the `CreatePluginObject` method pure virtual.

This means that we cannot construct a Scripted Interface base class
instance, so this patch also updates the various
`ScriptedInterpreter::CreateScripted*Interface` methods to return a
`nullptr` instead.`

This patch also removes the `ScriptedPlatformInterface` member from the
`ScriptInterpreter` class since it the interpreter can be owned by the
`ScriptedPlatform` instance itself, like we do for `ScriptedProcess`
objects.

Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>