{ "keys": ["j", "k"],       "args": {"key": "<esc>"},    "context": [{ "key": "setting.actual_intercept", "operand": true }], "command": "actual_keypress" },

That, in your Sublime keymap, should map jk to escape if you're using the ActualVim plugin.

On the backend, it's using NeoVim... So it's literally just vim. You can even use vim plugins. There really shouldn't be any missing features.