feat: add Microsoft Edge support to the Browser addon

[eliahu-frusin]

Description

Summary

The Browser addon (mitmproxy/addons/browser.py) supports launching an
isolated, proxy-configured instance of Chrome or Firefox via
browser.start, but had no equivalent for Microsoft Edge. Users without
Chrome or Firefox installed -- e.g. Windows users, where Edge is always
pre-installed -- had no working option.

Proposed in #8407 and confirmed welcome by maintainers before implementing.

Changes

• mitmproxy/addons/browser.py: added launch_edge(), following the exact
  same structure as launch_chrome() -- executable search paths across
  macOS/Windows/Linux, a Flatpak fallback (com.microsoft.Edge), and the
  same isolated --user-data-dir / --proxy-server launch flags (Edge is
  Chromium-based, so the same flags apply). Wired into the browser.start
  command dispatcher alongside the existing chrome/firefox branches.
• test/mitmproxy/addons/test_browser.py: added test_browser_start_edge
  and test_browser_start_edge_not_found, mirroring the existing Firefox
  test pattern.

Closes #8407

Checklist

• I have updated tests where applicable.
• I have added an entry to the CHANGELOG.