Update Reline core to stay in sync with the line_editor's completion_append_character

Description

When updating Reline.completion_append_character in Reline.completion_proc, the change does not get reflected until you enter a new Reline.readline or Reline.readmultiline context. It seems like the value of Reline.completion_append_character and Reline.line_editor.completion_append_character is out of sync.

Scenario: We are updating the value of Reline.completion_append_character = ' ' inside of the completion_proc, but it's not reflecting in the line_editor

Expected Behaviour

Being able to change Reline.completion_append_character in the completion proc and have it update the line_editor.

i.e. this code:

https://github.com/ruby/reline/blob/7534f7f92ad6ec5e38d7b404942cd7f30e3c1327/lib/reline.rb#L83-L93

To immediately update the line_editor's completion_append_character value

    def completion_append_character=(val)
      if val.nil?
        @completion_append_character = nil
      elsif val.size == 1
        @completion_append_character = val.encode(encoding)
      elsif val.size > 1
        @completion_append_character = val[0].encode(encoding)
      else
        @completion_append_character = nil
      end
      # Ensure the line_editor is updated too:
      line_editor.completion_append_character = @completion_append_character
    end

Current Behaviour

Changing Reline.completion_append_character in Reline.completion_append_character does not take effect until a new call to Reline.readline.

Terminal Emulator

MacOS

Reline Version

0.5.10

Start with the completion_append_character= implementation in lib/reline.rb at the linked lines, then reproduce the issue by changing the value inside a completion_proc. Verify how Reline.line_editor stores the value and confirm that the line editor reflects the new setting immediately, without requiring another readline or readmultiline context.