Better handling of text-edits at completion

[dimbleby]

The current implementation calls undo, but unless you happened to break the undo sequence before starting the completion - unlikely in regular typing - this will go very wrong:

• it'll undo more than just the insertion of the unwanted word (potentially a lot more)
• and having done that, it'll perform the text edit in the wrong place in the document

The approach here is borrowed from https://github.com/prabirshrestha/vim-lsp/blob/master/autoload/lsp/omni.vim#L321

• assume that the text edit is taking place at the completion point
• tweak that text edit so that it removes the now-unwanted completion

cf #662, #857 (comment)

[autozimu]

This is smart!

I will test a bit more before merging this in.

[autozimu]

Thanks!