Make element filtering configurable

[seanpdoyle]

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:

<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.

[seanpdoyle]

@keithamus could you review these changes?

[keithamus]

Looks like a sensible improvement. @github/web-systems need to review/merge this.

[theinterned]

@seanpdoyle thank you for the contribution!

[theinterned]

@seanpdoyle I just merged this, but realized that, like your other PRs it would be great to see

• tests
• the README updated to document this! 🙏 (I have merged Add function option documentation to README.md #52 so you can build on that)