[remote cli] support opening `file` URIs

<!-- ⚠️⚠️ Do Not Delete This! bug_report_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- 🕮 Read our guide about submitting issues: https://github.com/microsoft/vscode/wiki/Submitting-Bugs-and-Suggestions --> <!-- 🔎 Search existing issues to avoid creating duplicates. --> <!-- 🧪 Test using the latest Insiders build to see if your issue has already been fixed: https://code.visualstudio.com/insiders/ --> <!-- 💡 Instead of creating your report here, use 'Report Issue' from the 'Help' menu in VS Code to pre-fill useful information. --> Version: 1.53.2 (system setup) Commit: 622cb03f7e070a9670c94bae1a45d78d7181fbd4 Date: 2021-02-11T11:48:04.245Z Electron: 11.2.1 Chrome: 87.0.4280.141 Node.js: 12.18.3 V8: 8.7.220.31-electron.0 OS: Windows_NT x64 10.0.19041 Steps to Reproduce: 1. Connect to a WSL remote 2. Switch to the terminal 3. Run `$BROWSER 'file:///tmp'` Result:  Expected: Browser launching on Windows side with `file://///wsl$/Debian/tmp`. Curiously enough, running `$BROWSER 'file://///wsl$/Debian/tmp'` directly results in the `browser.sh` timing out with: ``` Unable to connect to VS Code server. Error in request ``` Opening `file://///wsl$/Debian/tmp` directly in the host's browser works just fine. Context: `mechanicalsoup` has a debug method that users python's `browser` module to open a browser with debug copy of downloaded page. Due to the above, can't use it easily right now :( <!-- 🔧 Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes <!-- 🪓 If you answered No above, use 'Help: Start Extension Bisect' from Command Palette to try to identify the cause. --> <!-- 📣 Issues caused by an extension need to be reported direct to the extension publisher. The …