implement scrubber for end user data in exceptions (#4999)

docs/api/scrubber.md

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+# Scrubber API
+
+When Slate throws an exception, it includes a stringified representation of the
+relevant data. For example, if your application makes an API call to access the
+child of a text Node (an impossible operation), Slate will throw an exception
+like this:
+
+```
+Cannot get the child of a text node: {"text": "This is my text node."}
+```
+
+If your rich text editor can include sensitive customer data, you may want to
+scrub or obfuscate that text. To help with that, you can use the Scrubber API.
+Here's an example of recursively scrubbing the `'text'` fields of any entity
+that gets logged.
+
+```typescript
+import { Scrubber } from 'slate'
+
+Scrubber.setScrubber((key, value) => {
+  if (key === 'text') return '... scrubbed ...'
+  return value
+})
+```
+
+By setting the scrubber in this way, the error example given above will be
+printed as
+
+```
+Cannot get the child of a text node: {"text": "... scrubbed ..."}
+```
+
+## Text Randomizer Example
+
+Here's an example "textRandomizer" scrubber, which randomizes particular fields
+of Nodes, preserving their length, but replacing their contents with randomly
+chosen alphanumeric characters.
+
+```typescript
+import { Scrubber } from 'slate'
+
+const textRandomizer = (fieldNames: string[]) => (key, value) => {
+  if (fieldNames.includes(key)) {
+    if (typeof value === 'string') {
+      return value
+        .split('')
+        .map(generateRandomCharacter)
+        .join('')
+    } else {
+      return '... scrubbed ...'
+    }
+  }
+
+  return value
+}
+
+const generateRandomCharacter = (): string => {
+  const chars =
+    'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLKMNOPQRSTUVWXYZ1234567890'
+  return chars.charAt(Math.floor(Math.random() * chars.length))
+}
+
+// randomize the 'text' and 'src' fields of any Node that is included in an
+// exception thrown by Slate
+Scrubber.setScrubber(Scrubber.textRandomizer(['text', 'src']))
+```
+
+In this example, a Node that looked like
+
+```json
+{ "text": "My test input string", "count": 5 }
+```
+
+will be logged by Slate in an exception as (the random string will differ):
+
+```json
+{ "text": "rSIvEzKe39l6rqQSCfyv", "count": 5 }
+```