Control strictness towards incoming lsp messages

Control strictness towards incoming lsp messages

A new variable, eglot-strict-mode controls whether Eglot is strict or
lax with regard to incoming LSP messages.

1. Bug reports should be tested with eglot-strict-mode set to
   '(disallow-non-standard-keys enforce-required-keys)

2. Users struggling to get non-standard servers working set this
   variable to '(), nil.  For now, by popular demand, this is the
   default value.

Note that this commit in particular introduces a new infrastructure,
but does not yet alter any code in Eglot to use it.  Neither is the
variable eglot--lsp-interface-alist populated.

* eglot-tests.el (eglot-strict-interfaces): New test.

* eglot.el (eglot--lsp-interface-alist): New variable.
(eglot-strict-mode): New variable.
(eglot--call-with-interface): New helper.
(eglot--dbind): New macro.
(eglot--lambda): New macro.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/144
GitHub-reference: per https://github.com/joaotavora/eglot/issues/156