fix: resolve #986 avoid wrong offset on Neovim for some postfix snippets

[Eric-Song-Nop]

resolve #986

According to lsp spec, additional text edit is not supposed to be used here.

/**

• An optional array of additional text edits that are applied when
• selecting this completion. Edits must not overlap (including the same
• insert position) with the main edit nor with themselves.
• Additional text edits should be used to change text unrelated to the
• current cursor position (for example adding an import statement at the
• top of the file if the completion item will insert an unqualified type).
  */

[Eric-Song-Nop]

Other uses of additional_text_edits should also be replaced gradually

[Eric-Song-Nop]

I am not sure if this is a common problem, so I am turning this pr in to a specific one for #986, thus the current implementation should be enough.