Intent to prototype and ship: HTMLInputElement showPicker() for textual input with datalist

[Vincent Hilla]

Summary: showPicker enables showing pickers for types such as file or color programmatically. We extend this to show the form autofill popup for single line text input types with a datalist.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1767228

Specification: https://html.spec.whatwg.org/multipage/input.html#dom-input-showpicker
- Which input types support a picker is implementation defined.

Platform coverage: Desktop, and Android once bug 1535985 resolves

Other browsers:
- Chrome shipped
- Safari shipped: https://bugs.webkit.org/show_bug.cgi?id=261701

web-platform-tests:

- There are more general tests in /html/semantics/forms/the-input-element/show-picker*

Example: https://demo.lukewarlow.dev/showPicker/