Display keybindings on different platform

[rebornix]

Review #702

In https://code.visualstudio.com/docs/editor/codebasics sometimes we are showing keybindings for different platforms

you can switch between them quickly by holding the Ctrl (Mac: 'Cmd') key

but sometimes we are smart enough to know that users are using Mac and we just show keys specific for Mac.

It's not a big deal but maybe we want to align them. Making it smart always.

[gregvanl]

We can use the platform specific key binding look up (command id -> keyboard shortcut) when the text is talking about a command which has a command id. When the text is mentioning a single key like cmd, we have to include both the Windows/Linux and Mac key since there is no command id to lookup.