Contributes to #139

This PR uses the [`remarkable`][remarkable] library to transform
Markdown into HTML instead of `marked`. While `marked` is a competent
library, it uses regular expressions to parse Markdown which include a
[syntax currently unsupported by the Hermes JS
engine](facebook/hermes#1027). In contrast,
`remarkable` does use any regular expressions to parse so it seems to
work just fine in React Native.

After switching to `remarkable`, using `<FhirValue>` in an Expo
application to display Markdown content in HTML resulted in a new error.
This turned to be a straightforward issue that the `dompurify` library
cannot sanitize an HTML string with access to the DOM. Thus, this PR
narrows HTML sanitizing to only be done when `dompurify` can do so.

## How can I test this change?

Create a new Expo app using the bonfhir template for it. Add a
`<FhirValue>` with some example Markdown content styled as HTML and
validate that the HTML shows up literally on the Android screen.

```tsx
const exampleMarkdown = "# Footer\n\nThis is a footer";

function ExampleScreen() {
  return <FhirValue
    options={{ style: "html" }}
    type="markdown"
    value={exampleMarkdown}
  />
}
```

The above should show something like:
```html
<h1>Footer</h1>
<p>This is a footer</p>
```

## Caveat

This helps make `@bonfhir/core` and `@bonfhir/react` more compatible
with React Native configured with Hermes by addressing specific issues.
Full Hermes compatibility is not confirmed yet however as testing the
complete surface of Bonfhir libraries and APIs is out of scope of this
fix. We welcome the community finding more compatibility issues however
that we can prioritize and address as needed!

[remarkable]: https://github.com/jonschlinkert/remarkable