Originally reported by @greazer

Webview interactive window had esc or ctrl+u to clear the input box contents. Now that the interactive window supports VS Code- and extension-contributed keymaps, this will cause keybinding conflicts. We have at least two options:

1. Similar to Fix history navigation in VS Code interactive window VSCodeVim/Vim#6980, add code to Vim extension to route to the VS Code clear command
2. Introduce an interactive.input.clear command that is by default not bound to any keymaps, and let users configure it themselves
3. Introduce the command and prompt the user somehow