Create terminal-core library for use in Code terminal plugin

I have searched open and closed issues for duplicates.

## Feature
The terminal plugin in Code does not have the same behaviour and features as the elementary Terminal app. For consistency it would be good if the same Terminal Widget and functionality could be used in both app.

**Describe the solution you'd like**
Create a Terminal core library containing the Terminal Widget and other necessary code which can be used be the Code terminal plugin and provide the same functionality.

**Describe alternatives you've considered**
The features desired could be provided by copying Terminal code into the Code terminal plugin.

**Additional context**
See https://github.com/elementary/code/issues/155