refactor: replace goto for Lua 5.1 compatibility

[malept]

👋 I tried out this plugin with Neovim nightly via the unstable PPA on a clean install of Ubuntu 20.04, and I got the following error on when I run lua require'completion'.on_attach():

E5108: Error executing lua error loading module 'completion.complete' from file 'completion-nvim/lua/completion/complete.lua':
completion-nvim/lua/completion/complete.lua:40: '=' expected near 'continue'

It turns out that the unstable PPA version of nvim is linked against Lua 5.1 (:lua print(_VERSION)), which does not have support for goto. This pull request refactors that part of the code so that it shouldn't need a goto. Please let me know if there are further adjustments I need to make (or feel free to do them yourself in the interest of speed).

[haorenW1025]

Oh I didn't actually know that... I used goto only for mocking the continue like stuff so it's fine. I'll stop abusing it in the future:) Also a side note is I don't know if the unstable PPA version contains all the feature we need(like tbl_filter). If still recommend building neovim from source since it shouldn't be so complicated to do that, thanks anyway:)