implement LSP-enum augend

[monaqa]

Closes #104, #106.

Problems

• Not working until language server is up and running
  • it is hard to solve essentially, but there is room for UX improvement, e.g. warning when server is not running
• Not intuitive what values are incremented to
• Slow with some language servers and condition
  • it might be better to assign this function to a key other than <C-a> / <C-x>