Check whether copilot is available

[ryoppippi]

Add check whether copilot is available.

[tmerse]

Should be merged, or people will be confused why Copilot still gives suggestion, even if disabled via config.

EDIT: Unfortunately, the patch does not seem to respect dynamically switching on/off via :Copilot enable/:Copilot disable.

For this to work, the vim.g.loaded_copilot == 1 and vim.b.copilot_enabled ~= false) would need to run every time before vim.fn['copilot#Complete'] is invoked I suppose.

EDIT 2: I found behavior around

vim.g.loaded_copilot
vim.b.copilot_enabled
vim.g.copilot_enabled

rather inconsistent in conjunction with :Copilot disable/enable. E.g. :lua print(vim.g.copilot_enabled) holds values of false, 0, 1.

What works for me is

source.is_available = function()
  return vim.g.copilot_enabled == 1
end

Of course ymmv.