Simplify eglot-format-buffer

Simplify eglot-format-buffer

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

* README.md (Commands and keybindings): Mention
eglot-format-buffer.

* eglot.el (eglot-format-buffer): Don't try to heuristically
preserve point here.
(eglot--apply-text-edits): Use replace-buffer-contents.

* eglot-tests.el (formatting): adjust test to strictly check for point
position.

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