Apply text edits as a single undoable edit

Apply text edits as a single undoable edit

As suggested by mkcms <k.michal@zoho.com>, but do it in
eglot--apply-text-edits, where it benefits all its users.

Also, just using undo-boundary is not enough, one needs
undo-amalgamate-change-group to mess with the boundaries already in
buffer-undo-list.

* eglot.el (eglot--apply-text-edits): Use
  undo-amalgamate-change-group.

GitHub-reference: close https://github.com/joaotavora/eglot/issues/22