Add mouse_select_option for native select dropdowns (#375)

[FazeelUsmani]

Fixes #375.

Native <select> dropdowns render their option list as a browser/OS-level popup that Playwright's synthetic mouse events can't reach, so mouse_click(x, y) on a <select> opens the dropdown but never picks an option.

Adds a mouse_select_option(x, y, options) action to the coord subset, mirroring the bid-based select_option. The lookup uses document.elementFromPoint and descends through open shadow roots and same-origin iframes (closed shadow roots and cross-origin frames are not reachable from JS). Wraps the call in call_fun(do, retry_with_force) for parity with the rest of the action set.

Tests cover top-level, same-origin iframe (via srcdoc=), and open shadow root.

Approach proposed by @imenelydiaker in the issue thread.