Add source:is_available API

lua/cmp/core.lua

@@ -59,7 +59,9 @@ core.get_sources = function(statuses)
     for _, s in pairs(core.sources) do
       if c.name == s.name then
         if not statuses or vim.tbl_contains(statuses, s.status) then
-          table.insert(sources, s)
+          if s:is_available() then
+            table.insert(sources, s)
+          end
         end
       end
     end

lua/cmp/source.lua

@@ -164,6 +164,14 @@ source.get_default_replace_range = function(self)
   end)
 end
 
+---Return the source is available or not.
+source.is_available = function(self)
+  if self.source.is_available then
+    return self.source.is_available()
+  end
+  return true
+end
+
 ---Get keyword_pattern
 ---@return string
 source.get_keyword_pattern = function(self)