Publish theme data to es6 modules

As a dev using a js-based build system
I can import a theme using es6 module syntax
so it fits with the rest of my dependencies

```
// Import just the parts I need, so tree-shaking can strip out unused data
import { resolvedRoles, metadata } from '@hig/theme-data/build/esm/lightGrayMediumDensityTheme';

// Import the entire theme as an object
import * as darkBlueTheme from '@hig/theme-data/build/esm/darkBlueHighDensityTheme';
```

---

*Acceptance*
Within the `build/es6` directory, write out one file for each theme. Each theme file should export `metadata`, `resolvedRoles`, and `unresolvedRoles`.

Start by inspecting the existing theme data outputs and the build/es6 directory to understand how theme files are organized. Add one ES6 module for each theme, then verify that every file exports metadata, resolvedRoles, and unresolvedRoles and supports the import forms shown in the issue.

Written by the indexing model from the issue text.