Improve ergonomics of Eglot's inlay hints

Improve ergonomics of Eglot's inlay hints

Instead of deleting inlay hints instantly as soon as the affected
changes, make hint overlays span one character (instead of being
length 0).  Give the overlays an "evaporate" property.

Given an inlay hints at position POS, its attached to [POS, POS+1] if
it's kind=1 (usually type hints) and [POS-1, POS] otherwise.  For
kind=1 hints, the 'cursor position of the first such overlay is also
tweaked, so that's it's less akward to edit around it.

* lisp/progmodes/eglot.el (eglot--before-change): Don't delete hints
  here.
  (eglot--update-hints-1): Rework.