Support search in iframe based webview

[mjbvz]

For #83188

Problem

On desktop, VS Code's webviews using the electron <webview> tag. This gives a nice api for searching inside a webview

However on web we use normal iframes for webviews. These currently don't support the search functionality because there is no equivalent API for searching inside a iframe.

This is one blocker for using normal iframes for the webviews in desktop VS Code: #83188

Request

Implement search for iframe based webviews. This should attempt to match the behavior of the existing search functionality as much as possible

/cc @deepak1556

[mjbvz]

Solutions investigated:

• Window.find — Non-standard. Doesn't provide enough control

• JavaScript library (such as mark.js) — Requires JS on page. Modifies page content

• Find text and other proposals — Never standardized and no longer active :(