Refactoring CoreRuntime and plugin (#247)

Refactoring CoreRuntime and plugin (#247)

* Pass apptype to plugin insteadof mode

The meaning of the mode that is delivered during plugin initialization is not clear.
And, VD plugin use mode to distinguish app-type.

So, change the parameter of plugin_initialize from mode to apptype.
Also, the script was modified to obtain the apptype in the candidate process.

The parameter of the CoreRuntime constructor was also unnecessary and deleted.

* Change CoreRuntime to static class

CoreRuntime is a wrapper class of .NET Runtime (coreclr),
So, that cannot be created with multiple instance, and cannot be called
multiple times.

So, I changed CoreRuntime to static class which contains only static functions.
Also, it was changed to explicitly call what was called as a destructor.

Additionally, some function name and file name have been changed.