feat: expose status getter

[Kaes3kuch3n]

When integrating auto-save functionality via this plugin's API, the only way to keep track of whether auto saving is enabled is by creating event listeners for the enable and disable event and tracking the state in an external field. To make state tracking a little easier, I've added an enabled() function for fetching the current state directly via the plugin's API.

Examle use case

Integration with snacks.toggle:

local autosave = require("auto-save")

require("snacks.toggle").new({
    name = "Auto Save",
    get = function()
       return autosave.enabled()
    end,
    set = function(state)
        if state then
            autosave.on()
        else
            autosave.off()
        end
    end,
}):map("<leader>uv")

[okuuva]

LGTM, thank you for the contribution! I think it would be good to include the snacks toggle example in the README. Can you add that as well?