Add support for when clause in commands

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FEATURE REQUEST:

Support "when" in custom binding commands. This will allow one to create toggle commands that do different things based on the state of the editor.

For example:

"vim.otherModesKeyBindingsNonRecursive": [
        {
            "before": ["-"],
            "commands": [
                {
                    "command": "workbench.files.action.focusFilesExplorer",
                    "when": "editorFocus",
                    "args": []
                },
                {
                    "command": "workbench.action.toggleSidebarVisibility",
                    "when": "explorerViewletFocus",
                    "args": []
                }
            ]
        }
    ]

Pressing - while focused on an editor, shows and sets focus to the file explorer. Pressing - in the file explorer, closes the explorer sidebar and sets you back into the editor.

Start by tracing how VSCodeVim parses and executes custom binding commands, using the example configuration as the behavioral reference. Verify that command entries can be selected by their when conditions, then test the toggle behavior in both editor focus and file explorer focus.