literate: add with-terminal, suspend-backend, resume-backend to org source

with-terminal macro was only in tangled .lisp (not .org). suspend-backend
and resume-backend generics + simple-backend methods + tests were also
in hand-edited .lisp only. All three added to org/backend-protocol.org
with proper prose, following the literate programming discipline.

Also added suspend/resume assertions to simple-backend-lifecycle test suite.

src/backend/package.lisp

@@ -19,9 +19,9 @@
    ;; Queries
    #:capable-p
    ;; Constructors
-   #:make-simple-backend
-   #:with-terminal
-   ;; Modern backend
+    #:make-simple-backend
+    #:with-terminal
+    ;; Modern backend
    #:modern-backend #:make-modern-backend
    ;; Detection
    #:detect-backend #:*detected-backend*