Setting to make the "Open in Browser" button and actions always use the external browser

## Request

Add a setting that makes the "Open in Browser" actions **always target the external browser** (the one configured in `workbench.externalBrowser`) instead of the Integrated Browser. Concretely:

- the globe button in the title bar (`workbench.browser.showInTitleBar`);
- the "Open in Integrated Browser" action in the editor title for `.html` files;
- links from the terminal, chat and other sources that today are governed by `workbench.browser.openLocalhostLinks`.

Something like:

```jsonc
// Which browser the "Open in Browser" button and actions use.
"workbench.browser.openTarget": "integrated" | "external"
```

Today a user who prefers their regular browser can only **hide** the title bar button and **disable** localhost interception. There is no way to keep the same one-click button and have it open Chrome/Edge/Firefox. The "Open in External Browser" item in the Integrated Browser's three-dot menu is a workaround, not a replacement: it requires opening the page in the Integrated Browser first, and for local `file://` pages it currently fails on Windows (#323733, root cause is the percent-encoded drive letter in the `file:` URL).

## Why

The Integrated Browser cannot replace a real browser for many workflows: extensions (React/Vue devtools, accessibility checkers), browser profiles with logged-in sessions, multi-engine testing, familiar DevTools. #316139 asks to change the *default*; this request is narrower and complements it: even with the default unchanged, let users retarget the existing button and actions once and keep using them.

## Related

- #316139 Turn off integrated browser for default (backlog)
- #323733 "Open in External Browser" fails with 0x2 for local files on Windows

VS Code 1.136.1 (a44adf7f53e00964ab890f9f8758a334f1fc15bc), Windows 11 10.0.26200.