Fix modifier keys being toggled when typing extended keys like arrows (#731)

* Fix modifier keys being toggled when typing extended keys like arrows

When sending keys via virtual key code, the KEYEVENTF_EXTENDEDKEY flag
was never set. Without this flag, Windows treats navigation keys (arrows,
Home, End, etc.) as numpad keys. Due to legacy numpad handling, Windows
internally injects fake modifier key-up/key-down events around numpad
keystrokes when a modifier is held, which causes the modifier state to
be toggled. This broke scenarios like Shift+Arrow for text selection.

Set KEYEVENTF_EXTENDEDKEY for known extended keys when sending via
virtual key code, matching how the scan code path already handles this.

Fixes #295

* Add test for text selection using Shift+Arrow via Keyboard.Pressing

Uses the repo's test applications (WinForms/WPF) to verify that
Pressing(LSHIFT) with arrow keys correctly selects text. Asserts by
typing over the selection and checking the resulting text.

Verifies the fix for #295.

* Update src/FlaUI.Core/Input/Keyboard.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/FlaUI.Core/Input/Keyboard.cs

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Roman <r.baeriswyl@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>