Support for VScode keybinding

[charlypa]

Phoenix should support Vs code key bindings

Add support for settings dropdown in shortcuts panel

The settings dropdown will have the following options

1. Reset to defaults
2. Use Phoenix Code Shortcuts
3. Use VS Code Shortcuts

creating the dropdown

• The code for the shortcuts extension is located in src/extensionsIntegrated/DisplayShortcuts folder.
• Dropdowns can be created using DropdownButton. See sample usage in FileFilters.js file.

[abose]

in keybindingManager.js, add a addExtensionKeyMap API that changes the _loadUserKeyMap function such that, we first take all the extension supplied keymaps and then apply the user keymap over it. This is the simplest way to add extension keymap support. We should also add a removeExtensionKeyMap as the _loadUserKeyMap API can be updated on the fly.