implemented xml type for new event system

src/Type/YText.js

@@ -1,4 +1,53 @@
+import ItemString from '../Struct/ItemString.js'
 import YArray from './YArray.js'
 
 export default class YText extends YArray {
+  constructor (string) {
+    super()
+    if (typeof string === 'string') {
+      const start = new ItemString()
+      start._parent = this
+      start._content = string
+      this._start = start
+    }
+  }
+  toString () {
+    const strBuilder = []
+    let n = this._start
+    while (n !== null) {
+      if (!n._deleted) {
+        strBuilder.push(n._content)
+      }
+      n = n._right
+    }
+    return strBuilder.join('')
+  }
+  insert (pos, text) {
+    this._y.transact(() => {
+      let left = null
+      let right = this._start
+      let count = 0
+      while (right !== null) {
+        if (count <= pos && pos < count + right._length) {
+          right = right._splitAt(this._y, pos - count)
+          left = right._left
+          break
+        }
+        count += right._length
+        left = right
+        right = right._right
+      }
+      if (pos > count) {
+        throw new Error('Position exceeds array range!')
+      }
+      let item = new ItemString()
+      item._origin = left
+      item._left = left
+      item._right = right
+      item._right_origin = right
+      item._parent = this
+      item._content = text
+      item._integrate(this._y)
+    })
+  }
 }