Eglot: fix M-x eglot-show-workspace-configuration (bug#61866)

Eglot: fix M-x eglot-show-workspace-configuration (bug#61866)

Now consult .dir-locals.el every time the workspace configuration is
needed:

- workspace/configuration server request
- workspace/didChangeConfiguration signal
- M-x eglot-show-workspace-configuration

The major-mode/hack-dir-local-variables-non-file-buffer trick is used.
When there is more than one, the server connection's "main" major mode
is used to find the relevant .dir-locals.el section.

* lisp/progmodes/eglot.el (eglot--lookup-mode): Fix docstring.
(eglot--connect): Simplify.
(eglot-show-workspace-configuration): Fix.
(eglot--workspace-configuration): Remove.
(eglot--workspace-configuration-plist): Rework.
(eglot-handle-request): Simplify.