feat: add filter option to sources (#1067)

* feat: add `source.filter` config

This allows the user to specify a `filter` function for each source,
like this:

```lua
-- don't show entries with kind "Text" from the "nvim_lsp" source

sources = {
{
  name = "nvim_lsp",
  filter = function(entry, ctx)
    local kind = types.lsp.CompletionItemKind[entry:get_kind()]

    if kind == "Text" then
      return true
    end
  },
}
```

By utilizing the `ctx` parameter, the user can also ignore certain
entries in certain contexts.

* fixup! feat: add `source.filter` config

* fixup! feat: add `source.filter` config

lua/cmp/source.lua

@@ -101,6 +101,8 @@ source.get_entries = function(self, ctx)
     return self.entries
   end)()
 
+  local entry_filter = self:get_entry_filter()
+
   local inputs = {}
   local entries = {}
   for _, e in ipairs(target_entries) do
@@ -115,7 +117,10 @@ source.get_entries = function(self, ctx)
     if e.score >= 1 then
       e.matches = match.matches
       e.exact = e:get_filter_text() == inputs[o] or e:get_word() == inputs[o]
-      table.insert(entries, e)
+
+      if entry_filter(e, ctx) then
+        table.insert(entries, e)
+      end
     end
   end
   self.cache:set({ 'get_entries', self.revision, ctx.cursor_before_line }, entries)
@@ -232,6 +237,16 @@ source.get_keyword_length = function(self)
   return config.get().completion.keyword_length or 1
 end
 
+---Get filter
+--@return function
+source.get_entry_filter = function(self)
+  local c = self:get_source_config()
+  if c.entry_filter then
+    return c.entry_filter
+  end
+  return function(_, _) return true end
+end
+
 ---Invoke completion
 ---@param ctx cmp.Context
 ---@param callback function