Add support for completing special references (e.g. buffers) in Eshell

Add support for completing special references (e.g. buffers) in Eshell

* lisp/eshell/em-cmpl.el (eshell-complete-parse-arguments): Handle
special references.

* lisp/eshell/em-arg.el (eshell-parse-special-reference): Ensure point
is just after the "#<" when incomplete, and handle backslash escapes
more thoroughly.
(eshell-complete-special-reference): New function.

* test/lisp/eshell/esh-arg-tests.el
(esh-arg-test/special-reference/default)
(esh-arg-test/special-reference/buffer)
(esh-arg-test/special-reference/special):
* test/lisp/eshell/em-cmpl-tests.el
(em-cmpl-test/special-ref-completion/type)
(em-cmpl-test/special-ref-completion/implicit-buffer)
(em-cmpl-test/special-ref-completion/buffer): New tests.