feat: Support new REST preview service

The REST preview service introduces its own preview token and API
endpoint for Live preview. This values will be used to fetch entry data
for live preview.

Replaced "api.contentstack.io" to "rest-preview.contentstack.com" and
replaced `management_token` to `preview_token`

fixes: https://contentstack.atlassian.net/browse/VC-311

Author: Deepak-Kharah

config.js

@@ -12,7 +12,7 @@ const config = {
     },
     live_preview: {
         enable: false,
-        host: 'api.contentstack.io'
+        host: 'rest-preview.contentstack.com'
     }
 };
 

index.d.ts

@@ -72,7 +72,7 @@ export interface ContentTypeCollection {
 
 export interface LivePreview {
     host: string
-    management_token: string
+    preview_token: string
     enable: boolean
 }