"open default keymap" should include vim and base keymaps, not just default keymap

[notpeter]

Steps to reproduce:

1. cmd+shift+p, "welcome toggle base keymap selector" -> select jetbrains
2. cmd+shift+p, "workspace: toggle vim mode" (unless already enabled)
3. cmd+shift+p, "zed: open default keymap"

Notice that you are seeing just the contents of default-macos.json but the contents of vim.json and macos/jetbrains.json are nowhere to be found.

Zed should render the combined version of these keymaps together all at once.