fix(core.lua): Defer vim.notify call in libuv eventloop using vim.sch…

[MustafaAamir]

…edule

nvim_echo's invocation - initially called in a fast-event context - is deferred using vim.schedule()

Fixes:

Lua callback:
vim/_editor.lua:0: E5560: nvim_echo must not be called in a fast event context
stack traceback:
        [C]: in function 'nvim_echo'
        vim/_editor.lua: in function 'notify'
        .../share/nvim/site/pack/core/opt/fff.nvim/lua/fff/core.lua:34: in function <.../share/nvim/site/pack/core/opt/fff.nvim/lua/fff/core.lua:29>