fix(logger): schedule vim.notify

[przepompownia]

Since neovim/neovim#31900 it's using nvim_echo which cannot be called in fast mode.

[przepompownia]

Not sure if we necessary need vim.in_fast_event() query here so not added it.

[mochaaP]

pls update the test helpers accordingly.

[przepompownia]

I see that in regal_spec

stub(vim, "schedule")
-- ...
    it("should log error for non-json output", function()
        local diagnostic = parser({ output = "non-json-output", err = "json error" })
        assert.same({}, diagnostic)
        assert
            .stub(vim.schedule)
            .was_called_with(function() vim.notify("[null-ls] non-json-output", vim.log.levels.ERROR, { title = "null-ls" }) end)
    end)

seems to be a step ahead but at the moment I don't know how to mock anonymous function or work it around in another way.

[przepompownia]

Thanks!