[api_summary] Track constant variable values

[kevmoo]

Currently, api_summary models top-level const variables and constant class fields as static getters, but drops the underlying constant expression/value. Modifying a const value can break consumers when used in compile-time constant contexts (such as annotations or switch cases).

Example

Changing:

const int maxRetries = 3;

to:

const int maxRetries = 4;

currently produces identical api_summary output.

[lrhn]

Makes sense for "simple" values, like integers and strings, or value objects (instances of value types that just carry their state).
Not so much for complex strategy objects.
If dart:convert changes the implementation of const Utf8codec utf8 = Utf8Codec(); to having an internal implementation ... = _Utf8Codec();, then it will behave the same, and nobody needs to know. But it is a different constant object.

(Also opens the question of when two constant objects from two different versions of the package are "the same".)