Fix: Prevent Shift key from intercepting shortcuts in Web Custom Editors

[Tejas-Raj01]

Fixes #322505

Description

This PR fixes an issue in the Web environment where the Shift key incorrectly intercepts keyboard shortcuts in Custom Editors.

Previously, holding the Shift key would trigger the alternative actions state (toggling toolbar icons), which prevented standard shortcuts involving the Shift key (like Shift+Cmd/Ctrl+V for toggling Markdown preview) from executing properly in web custom editors. This fix ensures that the alternative action state bypasses the Shift key check when running in the Web environment (isWeb).

How to test

1. Open VS Code in a web environment (e.g., ./scripts/code-web.sh or vscode.dev).
2. Open a Markdown file and its custom preview.
3. Press a keyboard shortcut that includes the Shift key (like Shift+Cmd+V on Mac or Shift+Ctrl+V on Windows/Linux).
4. Verify that the shortcut successfully toggles the preview editor instead of merely toggling the alternative actions in the editor toolbar.

[Copilot]

Pull request overview

This pull request addresses a VS Code for the Web issue where Shift was treated like an “alternative actions” modifier for custom editor toolbars/menus, which could interfere with normal Shift+… keybindings (e.g. Markdown preview toggle).

Changes:

• Disable using Shift as an alternative-actions modifier when running in the Web environment (isWeb) for context menu action resolution.
• Disable using Shift (while hovered) to toggle an action’s alternate command in the Web environment, leaving Alt as the modifier.