Expose mergeConfigs and Configs from dist-modern

[BPScott]

These two exports are part of the public api, consumed by polaris-react
Lets expose them at the top level of modern so apps don't have go reach
deep

This helps set us up for the future where we switch importing @shopify/polaris-tokens to look at the modern stuff instead of pulling in legacy things.

Once this is merged and released we can update polaris-react to stop it reaching deep:

- import {mergeConfigs} from '@shopify/polaris-tokens/dist-modern/utils';
+ import {mergeConfigs} from '@shopify/polaris-tokens/dist-modern';

// and

- import type {Config} from '@shopify/polaris-tokens/dist-modern/types';
+ import type {Config} from '@shopify/polaris-tokens/dist-modern';

[kaelig]

That makes sense!

Can you add an item to CHANGELOG.md?