Allow extensions to contribute a default when clause to be used when a user add a key binding for the command

Related: https://github.com/microsoft/vscode/issues/40389#issuecomment-738419359 Commands today can often have complex `when` clauses for the conditional for when they should apply, but these conditions aren't able to be expressed to the user when they add a key binding. Maybe we could add a `defaultWhen` or `keybindingWhen` (or something like that) property to commands in the `package.json`, so that when a user add a key binding for that command, the `when` clause would be populated with that value. Alternatively, I was thinking that we could re-use the `when` clause of the command's `commandPalette` contribution to get at a reasonable default, but it might be tricky and we'd need to discount any `false` clauses. Another option (though larger in scope of what I am proposing/wanting here) would be to allow having an entry in the `keybindings` contribution point without the actual `key`, so that it would be used as the "template" for when a user adds a binding. This could also double as a way for extensions to opt-out commands from key bindings altogether, by setting `"when": false` like the `commandPalette` contribution point. As there are plenty of commands that an extension can have that they can't really support a user adding a keybinding for. But that 2nd point, is I think out of scope of this issue. /cc @alexdima, @jrieken