fix(developer): prevent opening .kpj in multiple processes

Fixes #10003.

Multiple small fixes required here:
* Add .kpj as an editable file type
* Ensure that when OpenFile returns nil (which it always will for a
  .kpj), that we don't crash
* Ensure that projects open in a new process rather than replacing the
  current project session

Adds a TODO for New Project -- this should be opened in a new process if
we already have a project open in the current process. Will push this
into a separate PR.