Eglot: simplify inlay hints implementation with jit-lock

Eglot: simplify inlay hints implementation with jit-lock

This implementation is much simpler than the one based on
windows-scroll-functions.  It's also supposedly safer, as long as
jit-lock guarantees refontification of affected regions.

It's not _trivially_ simple though, as simply adding
'eglot--update-hints-1' to jit-lock-functions, while possible, is
going to request inlay hints from the LSP server for many small
regions of the buffer, depending on what jit-lock thinks is best.  So
we keep coalescing these into a larger region until the time is
suitable for a more bandwidth-efficient request.

To do this, we use a jit-lock implementation detail,
jit-lock-context-unfontify-pos, which is a proxy for knowing that the
jit-lock-context-timer has run.  Not sure how brittle it is, but it
seems to work reasonably.

We also get rid of the previous "get hints for entire buffer"
implementation.

* doc/misc/eglot.texi (Eglot Variables): Remove mention
to deleted eglot-lazy-inlay-hints.

* lisp/progmodes/eglot.el (eglot-lazy-inlay-hints)
(eglot--inlay-hints-after-scroll)
(eglot--inlay-hints-fully)
(eglot--inlay-hints-lazily): Remove.
(eglot--update-hints): Add function.
(eglot-inlay-hints-mode): Simplify.