Improved messages and fixed errors

The following improvements were made to messages and errors shown in the extension:

• Added a notification that appears when selecting the multi-core SoC to build targets that have an _ns variant. The notification appears when you select the build target version without _ns and invites you to consider using the _ns variant.

• Added a warning in the Status Bar whenever an SDK or toolchain is not selected. You can hover over the item to check the current configuration. The extension will default to the last installed SDK and toolchain version.

• Updated the notification about the invalid toolchain installation for cases when SEGGER J-Link is not installed.

• Updated how the terminal output of nrfjprog is displayed, so that it now includes line separation to make the output clearer.

• Fixed an issue where the extension would not validate the directory path when adding an existing application. This led to issues if the path was changed outside of VS Code. Now, the extension will run a path validation check when adding an existing application just as it does when creating a new application.