Implement the Synonym process plugin

Problem/Motivation

The Synonym process plugin should merge values that will be used in the entity reference context. The use case is covered when these values are describing the same reality and we want to define them as the same taxonomy term, this can be also useful to cover typos.

Proposed resolution

Make use of the entity_lookup process plugin that is already covered by the Migrate Plus module. Use a yml file with the desired term as key and synonym strings as value.

Remaining tasks

• Implement
• Documentation
• Unit tests