improve docs code blocks highlighting (#2049)

Summary:
This PR adds missing Objective-C entry to the Docusaurus config, fixes Objective-C code blocks label and adds or replaces several code block labels to improve the currently highlighted blocks.

Prism in Docusaurus by default also includes syntax highlight for `jsx` and `tsx`, which improves the nodes and props highlight, so I have used those syntaxes in few places too.

I have also fixed one typo that I have spotted and my IDE made a cleanup of whitespaces in edited files.

## Changelog

* [website] improve docs code blocks highlighting

Pull Request resolved: https://github.com/facebook/flipper/pull/2049

Test Plan:
The changes have been tested running Flipper website on `localhost`.

## Preview

<img width="650" alt="Screenshot 2021-03-12 150934" src="https://user-images.githubusercontent.com/719641/110951135-fff20d00-8344-11eb-96db-1bdc82c8d5ea.png">

<img width="649" alt="Screenshot 2021-03-12 151022" src="https://user-images.githubusercontent.com/719641/110951268-2ca62480-8345-11eb-9d3b-1a48f1267776.png">

Reviewed By: priteshrnandgaonkar

Differential Revision: D27336599

Pulled By: passy

fbshipit-source-id: c2dfb3d8cad4675da0f5e1270cada1e56a0175c0

docs/extending/desktop-plugin-structure.mdx

@@ -122,7 +122,7 @@ Flipper Desktop plugins come in three possible flavors:
 
 A plugin always exposes two elements from its entry module (typically `src/index.tsx`): `plugin` and `Component`:
 
-```typescript
+```tsx
 import {PluginClient} from 'flipper-plugin';
 
 export function plugin(client: PluginClient) {
@@ -143,7 +143,7 @@ Flipper also supports so-called device plugins - plugins that are available for
 so are a bit more limited in general.
 Their entry module anatomy is:
 
-```typescript
+```tsx
 import {DevicePluginClient} from 'flipper-plugin';
 
 export function devicePlugin(client: DevicePluginClient) {