Add eglot-diagnostics-hook, and use it for flymake

[purcell]

We provide a hook that is called whenever diagnostics are
received, and each hook function will be passed the diagnostics. This
allows clean extension to support flycheck or other mechanisms for
diagnostics display, while maintaining the existing flymake support
intact.

See #596

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A couple of design notes:

• Diagnostics passed to hook functions are still flymake diagnostics, but that seems okay to me: it's as good a format for structured diagnostics as any. The alternative would be to pass the raw diag-specs and decode them in the hook functions, which is probably overthinking things at this point.
• Flycheck can handle diagnostics for "other files" and display them sanely, and it's not clear how/if this would fit into the scheme.