feat(opts): `executor` could be a string

[cpea2506]

As a user, I prefer setting config without the need to know which I should require. I wonder if this would be considered as a breaking change.

Example config

require("rust-tools").setup {
    tools = {
        executor = "toggleterm",
    }
}

[IndianBoy42]

I think the old behaviour is useful too, the user can extend it themselves to add other executors based on whatever terminal plugins they like (or something else)

You can check if type(executor) == "string" and use an inbuilt executor if it exists