feat: add getText() and generateText() methods (fix #1428) (#1875)

* move getTextBetween method

* add getText method

* refactoring

* refactoring

* refactoring

* move renderText to schema, add generateText method

* add GenerateText demo

* docs: update

* remove demo from html page

packages/core/src/helpers/getText.ts

@@ -0,0 +1,18 @@
+import { TextSerializer } from '../types'
+import { Node as ProseMirrorNode } from 'prosemirror-model'
+import getTextBetween from './getTextBetween'
+
+export default function getText(
+  node: ProseMirrorNode,
+  options?: {
+    blockSeparator?: string,
+    textSerializers?: Record<string, TextSerializer>,
+  },
+) {
+  const range = {
+    from: 0,
+    to: node.content.size,
+  }
+
+  return getTextBetween(node, range, options)
+}