add a mira observable summary section for amr-to-mmt task

[mwdchang]

Summary

MMT's observable contains an expression string that is a bit more difficult to work with in javascript space, we will add a transformer/summarizer at the task runner stage to make downstream consumption a little easier.

TLDR: This is effectively a subset of the mmt.observables section, but with better access to linked states.

The new object defn at hand,

export interface ObservableSummary {
       [key: string]: {
               name: string;
               display_name: string;
               description: string;
               expression: string;
               references: string[];
       };
}

Testing

amr-to-mmt should have a new section for obsrvable summary