Make element filtering configurable

Introduce support for a `{ storageFilter: (field) => boolean }`
configuration override for calls to `persistResumableFields`. By
default, preserve the existing behavior of rejecting fields where
[value][] is equivalent to [defaultValue][].

This change is motivated by situations where an element's value is
impacted by events or packages that change its `[value]` attribute
directly.

For example, consider the following sample code:

```html
<input id="input" type="hidden" value="the default value">
<output id="output"></output>

<button>Change</button>

<script>
  const button = document.querySelector("button")
  const input = document.getElementById("input")
  const output = document.getElementById("output")

  output.textContent = input.defaultValue

  button.addEventListener("click", () => {
    input.setAttribute("value", "a new default value")
    output.textContent = input.defaultValue
  })
</script>
```

You can experiment with the code on [JSFiddle][].

Clicking the button changes the `<input>` element's `[value]` directly,
which has a side-effect of change its `.defaultValue` as well.

Within the context of `session-resume`, this means that any elements
impacted by side-effects of other code on the page will always be
omitted from being stored.

This is especially incompatible with `<trix-editor>` elements provided
by [Trix][] and [Action Text][].

[Trix]: https://trix-editor.org
[Action Text]: https://edgeguides.rubyonrails.org/action_text_overview.html

[value]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement#value
[defaultValue]: https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement#defaultvalue
[JSFiddle]: https://jsfiddle.net/1nwb4o23/

src/index.ts

@@ -1,20 +1,28 @@
 // Last submitted HTMLFormElement that will cause a browser navigation.
 let submittedForm: HTMLFormElement | null = null
 
-function shouldResumeField(field: HTMLInputElement | HTMLTextAreaElement): boolean {
-  return !!field.id && field.value !== field.defaultValue && field.form !== submittedForm
+function shouldResumeField(field: HTMLInputElement | HTMLTextAreaElement, filter: StorageFilter): boolean {
+  return !!field.id && filter(field) && field.form !== submittedForm
 }
 
+function valueIsUnchanged(field: HTMLInputElement | HTMLTextAreaElement): boolean {
+  return field.value !== field.defaultValue
+}
+
+type StorageFilter = (field: HTMLInputElement | HTMLTextAreaElement) => boolean
+
 type PersistOptions = {
   selector?: string
   keyPrefix?: string
   storage?: Pick<Storage, 'getItem' | 'setItem'>
+  storageFilter?: StorageFilter
 }
 
 // Write all ids and values of the selected fields on the page into sessionStorage.
 export function persistResumableFields(id: string, options?: PersistOptions): void {
   const selector = options?.selector ?? '.js-session-resumable'
   const keyPrefix = options?.keyPrefix ?? 'session-resume:'
+  const storageFilter = options?.storageFilter ?? valueIsUnchanged
 
   let storage
   try {
@@ -33,7 +41,7 @@ export function persistResumableFields(id: string, options?: PersistOptions): vo
     }
   }
 
-  let fields = resumables.filter(field => shouldResumeField(field)).map(field => [field.id, field.value])
+  let fields = resumables.filter(field => shouldResumeField(field, storageFilter)).map(field => [field.id, field.value])
 
   if (fields.length) {
     try {