Don't insert prompt input before markers in ERC

Don't insert prompt input before markers in ERC

* etc/ERC-NEWS: Update entry in 5.6 section announcing a persistent
prompt.
* lisp/erc/erc-dcc.el (erc-dcc-chat-setup): Use helper
`erc--initialize-markers' to set up prompt.
* lisp/erc/erc.el (erc--initialize-markers): Change
`erc-insert-marker' type to t.  Do this to spare modules that want to
remember the current message location from resorting to workarounds,
like leaving a marker one char before the end of a message, which is
not foolproof.
(erc--refresh-prompt): Temporarily change type of `erc-insert-marker'
when inserting prompt.
(erc-display-line-1): Insert incoming message normally instead of
forcing before markers, thus respecting markers left by third parties.
Strictly speaking, this is a breaking change.
(erc-display-msg): Insert normally instead of calling
`insert-before-markers', which was a regression introduced by 05f6fdb9
"Preserve ERC prompt and its bounding markers".
* test/lisp/erc/erc-dcc-tests.el
(erc-dcc-tests--dcc-handle-ctcp-send,
erc-dcc-tests--erc-dcc-do-GET-command): Use helper to initialize
prompt and markers.
* test/lisp/erc/erc-networks-tests.el
(erc-networks--ensure-announced,
erc-networks--rename-server-buffer--existing--reuse,
erc-networks--rename-server-buffer--existing--noreuse,
erc-networks--rename-server-buffer--reconnecting,
erc-networks--rename-server-buffer--id,
erc-networks--rename-server-buffer--existing--live,
erc-networks--rename-server-buffer--local-match,
erc-networks--rename-server-buffer--local-nomatch): Use helper to
initialize prompt and bounding markers.
* test/lisp/erc/erc-stamp-tests.el (erc-stamp-tests--insert-right):
Use `erc--initialize-markers' for setting up prompt.
* test/lisp/erc/erc-tests.el (erc--refresh-prompt): Adjust assertions
for expected prompt content.  (Bug#60936)