Add converter for TSMappedType

I am using XOR type from here: https://bitbucket.org/atlassian/atlaskit-mk-2/src/31638e072721303cafbea43fe78c94a1ef92fd11/packages/core/type-helpers/src/index.ts#lines-159
To specify a type that is one or another, but not both here:
https://bitbucket.org/atlassian/atlaskit-mk-2/pull-requests/6112/ms-1987-step-31-move-editor-and-other/diff#Lpackages/editor/editor-core/src/plugins/media/types.tsT79

Basically, I want this prop to accept either
```
viewMediaClientConfig: MediaClientConfig;
```
or
```
viewContext: Promise;
```
but not both.