Add a global mechanism for refering to actions

[zadjii-msft]

Spec: #6902, #6899 - Action IDs.md

As we come to rely more on actions being a mechanism by which the user defines
"do something in the Terminal", we'll want to make it even easier for users to
re-use the actions that they've already defined, as to reduce duplicated json as
much as possible. This spec proposes a mechanism by which actions could be
uniquely identifiable, so that the user could refer to bindings in other
contexts without needing to replicate an entire json blob.

[DHowett]

I don't know what milestone to put this into, but since there's a spec I'm inclined to think 2.0?