Stop plugin writing files into every folder when enabled at user level

Right now the plugin will write the `selene.toml` and definition files into the workspace directory, every time you open one if the plugin is enabled globally.

We should instead check for some condition first.

Brief discussion with @Rider-Linden led to tentative decision to trigger the file writing on a successful match of a temp file to a source file in the project.

Any other suggestions?