Add benchmark for text insertion

perf/benchmarks/insert-text-normal/index.js

@@ -0,0 +1,14 @@
+
+module.exports = {
+  setup(state) {
+    // Move cursor
+    return state.transform()
+      .collapseToStartOf({ key: '_cursor_' })
+      .moveForward(10) // Move inside the text
+      .apply()
+  },
+
+  run(state) {
+    return state.transform().insertText('a').apply()
+  }
+}

perf/benchmarks/insert-text-normal/input.yaml

@@ -0,0 +1,41 @@
+nodes:
+- kind: block
+  type: paragraph
+  nodes:
+  - kind: text
+    ranges:
+    - text: 'This is editable '
+    - text: 'rich'
+      marks:
+      - type: bold
+    - text: ' text, '
+    - text: 'much'
+      marks:
+      - type: italic
+    - text: ' better than a '
+    - text: '<textarea>'
+      marks:
+      - type: code
+    - text: '!'
+- kind: block
+  key: _cursor_
+  type: paragraph
+  nodes:
+  - kind: text
+    ranges:
+    - text: 'Since it''s rich text, you can do things like turn a selection of text '
+    - text: 'bold'
+      marks:
+      - type: bold
+    - text: ', or add a semantically rendered block quote in the middle of the page,
+        like this:'
+- kind: block
+  type: block-quote
+  nodes:
+  - kind: text
+    text: 'A wise quote.'
+- kind: block
+  type: paragraph
+  nodes:
+  - kind: text
+    text: 'Try it out for yourself!'