Introduce `source` completion mode

[luozhiya]

Let fittencode.nvim serve as a source for nvim-cmp

• [x] Can work when ['<cr>'] = cmp.mapping.confirm({ select = false }),
• [ ] Can work without ['<cr>'] = cmp.mapping.confirm({ select = false }),
  • Require confirm to submit multiple lines of text at once
• [x] Implementing through the insertText
• [ ] Implementing through the additionalTextEdits?
  • Need update cursor
• [ ] Can be triggered by any character
  • get_trigger_characters returns a set and cannot implement any key
• [x] Support any UTF-8 character
• [x] The document window supports syntax highlighting
• [x] Update document