Fix: Update markdown-to-jsx (styleguidist#1599)

This updates the markdown-to-jsx package to v6.11.4 in order to patch a security vulnerability as reported at styleguidist#1596 and addressed here at quantizor/markdown-to-jsx#306

Fix: Fix navigation to sections with numbers in the name (styleguidis…

…t#1595)

Fixes styleguidist#1594

Fix: Show examples with targetIndex = 0 (styleguidist#1573)

Co-authored-by: Vladimir Akimov <[email protected]>

Fix: Fix missing FlowType enum values in prop description (styleguidi…

…st#1571)

In `ef4c109b`, the file `PropsRenderer.js` (located at
`src/client/rsg-components/Props/PropsRenderer.js`) was removed. In
`PropsRenderer.js`, the `renderExtra` method checked whether `getType`
for the argument to `renderExtra` was present:

```es6
function renderExtra(prop) {
  const type = getType(prop);
  if (!type) {
    return null;
  }
  ...
}
```

However, in `ef4c109b`, this method was replaced with `renderExtra.tsx`
and the condition was changed to:

```typescript
export default function renderExtra(prop: PropDescriptorWithFlow): React.ReactNode {
  const type = getType(prop);
  if (!prop.type || !type) {
    return null;
  }
```

Unfortunately, this extra condition has resulted in this method always
returning `null` for a Flow typed prop as `prop.type` is always `null`
as `prop.type` is never set.

This commit reverts the condition to what it was before the migration to
TypeScript.

Fix: Correct Usage of CleanWebpackPlugin (styleguidist#1565)

I was still using an option that no longer exists. Fixed to mimic the original behavior. The option `root` was removed in v2, but wasn't changed during the upgrade in styleguidist@9804259

It's deleting everything in the styleguidistDir folder which isn't the way it was before.
After this change it only deletes files in build/.

Fix: Sections with href were always open in new tab, regardless of th…

…e external option (styleguidist#1564)

Fixes styleguidist#1559

Fix: Prop types are missing for TypeScript (styleguidist#1563)

Closes styleguidist#1551

Fix: Exported TypeScript typings (styleguidist#1550)

Breaking: Drop Node.js 8 support (styleguidist#1545)

The minimum supported Node.js version is 10.

Fix: Open all sections when searching (styleguidist#1537)

When searching with the option tocmode: 'collapse' the sections do not open automatically (because they keep their state). The found section or page is therefore not visible nor clickable.