Eglot: implement inlay hints (bug#61412, bug#61066)scratch/eglot-inlay-hints

Inlay hints are small text annotations to specific parts of the whole
buffer, not unlike diagnostics, but designed to help readability
instead of indicating problems.  For example, a C++ LSP server can
serve hints about positional parameter names in function calls and a
variable's automatically deduced type.  Emacs can display these hints
in many little 0-length overlays with an 'before-string property, thus
helping the user not have to remember these things by heart.

Since inlay hints are potentially a large amount of data to request
from the LSP server, the implementation strives to be as parsimonious
as possible with these requests.

So, by default, inlay hints are only requested for the visible
portions of the buffer across windows showing this buffer.  This is
done by leveraging the 'window-scroll-functions' variable, making for
a reasonably complex implementation involving per-window timers.  When
scrolling a window, it may take a short amount of time for inlay hints
to "pop in".  The new user variable 'eglot-lazy-inlay-hints' can be
used to exert some control over this.

Specifically, if the variable's value is set to 'nil', then inlay
hints are greedily fetched for the whole buffer every time a change
occurs.  This is a much simpler mode of operation which may avoid
problems, but is also likely much slower in large buffers.

Also, by default this inlay feature is turned ON, which is the usual
practice of Eglot when facilities are in place to enable a given
feature.  Although this can be tweaked by the user via
'eglot-ignored-server-capabilities' as usual, it's possible this
feature is visually suprising enough to warrant an exception to the
usual rule.

I haven't tested inlay hints extensively across many LSP servers, so I
would appreciate any testing, both for functional edge cases and
regarding performance.  There are possibly more optimization
oportunities in the "lazy" mode of operation, like more aggressively
deleting buffer overlays that are not in visible parts of the buffer.

Though I ended up writing this one from scratch, I want to thank
Dimitry Bolopopsky <dimitri@belopopsky.com> and Chinmay Dala
<dalal.chinmay.0101@gmail.com> for suggestions and early patches.

* lisp/progmodes/eglot.el (eglot--lsp-interface-alist): Define
InlayHint.
(eglot-client-capabilities): Announce 'inlayHint' capability.
(eglot-ignored-server-capabilities): Add :inlayHintProvider.
(eglot--document-changed-hook): New helper hook.
(eglot--after-change): Use it.
(eglot-inlay-hint-face, eglot-type-hint-face)
(eglot-parameter-hint-face): New faces.
(eglot--update-hints-1, eglot--inlay-hints-after-scroll)
(eglot--inlay-hints-fully, eglot--inlay-hints-lazily): New helpers.
(eglot-lazy-inlay-hints): New user variable.
(eglot-inlay-hints-mode): New minor mode.
(eglot--maybe-activate-editing-mode): Try to activate
eglot-inlay-hints-mode.
(eglot--before-change): Remove overlays immediately in the
area being changed.

* doc/misc/eglot.texi (Eglot Features): Mention inlay hints.
(Eglot Variables): Mention eglot-lazy-inlay-hints.

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