Extend composition patterns selectively

> A lemma for this might be the ability to "add connection" where you take two variables and add a junction coupled to both of them.

_Originally posted by @jpfairbanks in [#36](https://github.com/AlgebraicJulia/DiagrammaticEquations.jl/issues/36#issuecomment-2155161672)_

This is in the context of having multiple Decapodes, which are to be composed by finding identical names, but also identifying certain variables which do not have matching names. The workflow thus becomes:
1. Generate default composition diagram.
2. Tweak by adding in a new connection.