Support for exports analysis

[guybedford]

The current specification defines imports and metadata for a module source but not the exports listing, which is currently tied directly to the source text module record, as opposed to being something that can easily be specified generally for any cyclic module record.

We need to (1) determine how important exports analysis data is, and whether this must still be prioritised. Then depending on the result of that we can either:

a) Continue to omit specifying exports
b) Specify exports only for ModuleSource.prototype.exports() JS module source export data and not AbstractModuleSource.prototype.exports() generic module source export data.
b) Specify AbstractModuleSource.prototype.exports() in a way that allows each concrete module record to define its own host-defined implementation while conforming to the expected signature convention.

(1) and the needs here should drive the decision on which path to take further.