[lldb/Plugins] Move ScriptedThreadInterface to ScriptedThread

Since we can have multiple Scripted Threads per Scripted Process, having
only a single ScriptedThreadInterface (with a single object instance)
will cause the method calls to be done on the wrong object.

Instead, this patch creates a separate ScriptedThreadInterface for each
new lldb_private::ScriptedThread to make sure we interact with the right
instance.

rdar://87427911

Differential Revision: https://reviews.llvm.org/D117070



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>