WIP: pcase macro

[mkcms]

Previously we were strict about the structure and contents of LSP objects. This led to errors when certain servers sent messages with fields not defined in the specification (because those fields are used by other clients, or the protocol evolved and added some fields and we never upgraded).

This commit makes eglot tolerate unknown keys in all incoming messages.

• eglot.el (eglot--destructuring-bind): New macro. (eglot-handle-notification): (eglot--register-unregister): (eglot--xref-make): (xref-backend-apropos): (eglot-completion-at-point): (eglot--sig-info): (eglot-help-at-point): (eglot--apply-workspace-edit): (eglot-code-actions): (eglot--register-workspace/didChangeWatchedFiles): Use it instead of cl-destructuring-bind.