Eglot: in eglot-code-action-indications, make 'margin' interactive

Eglot: in eglot-code-action-indications, make 'margin' interactive

Didn't know it was possible, thanks to Ergus <spacibba@aol.com> for the
tip about binding [left-margin mouse-2] in the keymap.

The technique adopted in 'eglot-mouse-call' possibly still doesn't work
very well if the margin thing we're clicking on is not on the line point
is currently on.  But we don't have that problem (yet?) because LSP
action suggestions function mostly on a "at point" basis.

* lisp/progmodes/eglot.el (eglot-code-action-indications): Adjust docstring.
(eglot--mouse-call): Don't go anywhere except in text area.
(eglot-diagnostics-map): Bind eglot-mouse-actions to left-margin.

(cherry picked from commit e2ff77052da91e662bbd3cb19a742604db742d32)