vim.deprecate should not be used until neovim 0.9.0

[wookayin]

vim.deprecate is not available yet as of neovim 0.8.0. The use of it
(introduced in #35) will break stable versions of neovim.

[wookayin]

For neovim <= 0.8.0, it will have no deprecation warnings but at the moment it will be fine. But if you would like to have deprecation warnings printed even before 0.9.0 (i.e., including the stable versions), we can use some other ways like simple vim.notify or print. That said, it's better have the plugin NOT broken than having no messages...

[hrsh7th]

LGTM.,Thank you!

[wookayin]

Correction: vim.deprecate is introduced in and available in 0.8.0 ( neovim/neovim@73741e9); I thought it was since 0.9.0-nightly because helpful.vim reports so. Anyway it would be good to maintain some backward compatibility for a while (e.g., nvim-cmp's minimum supported version is still 0.7.0).