Add option to show visual erc-keep-place indicator

Add option to show visual erc-keep-place indicator

* lisp/erc/erc-goodies.el (erc-keep-place-indicator-style,
erc-keep-place-indicator-buffer-type,
erc-keep-place-indicator-follow): New options for anchoring kept place
visually.
(erc-keep-place-indicator-line, erc-keep-place-indicator-arrow): New
faces.
(erc--keep-place-indicator-overlay): New internal variable.
(erc--keep-place-indicator-on-window-configuration-change): New
function to subscribe to `window-configuration-change-hook' and maybe
update kept-place indicator.
(erc--keep-place-indicator-setup): New function to initialize buffer
for local module `keep-place-indicator'.
(erc-keep-place-indicator-mode, erc-keep-place-indicator-enable,
erc-keep-place-indicator-disable): New local ERC module.  Depends on
"parent" module `keep-place'.  Like `fill-wrap', this is (for now)
also deliberately left out of the widget menu for `erc-modules'.
(erc-keep-place-move, erc-keep-place-goto): Add new commands for
manually updating and jumping to keep-place indicator.
(erc-keep-place): Move `erc--keep-place-overlay' when applicable.
* test/lisp/erc/erc-goodies-tests.el (erc-keep-place-indicator-mode):
Add test.
(Bug#59943.)