Add analyzer to extract environment variables used by code

[zomglings]

Each environment variable should reference the definition it is used inside (if one exists). Otherwise, the reference should be null.

[zomglings]

The way I'm planning to do this is to add two types of metadata: dependency and invocation.

dependency will be used for things like #defines, imports and requires.

invocation will resolve the maximally qualifiable names of symbols used in the code as variables, functions, etc. Qualification will be performed relative to dependencies.