Simplify option erc-fill-wrap-merge-indicator

Simplify option erc-fill-wrap-merge-indicator

* lisp/erc/erc-fill.el (erc-fill-function): Add hyperlink to
`erc-fill-wrap-mode' in doc string.
(erc-fill-wrap-merge-indicator-face): New face.  The rationale for
adding this is that hard-coding `erc-fill-wrap-merge-indicator' to use
specific non-ERC faces forces folks to customize the option by
specifying a value manually, which may not be easy for new users,
seeing as its :type is relatively complex.
(erc-fill-wrap-merge-indicator): Add new preset without leading
space, for narrow windows, and replace `shadow' with
`erc-fill-wrap-merge-indicator-face' everywhere.
(erc-fill-wrap-mode): Add configuration hint to doc string for related
options when facing narrow windows.
(erc-fill--wrap-insert-merged-post, erc-fill--wrap-insert-merged-pre):
Fix sloppy thinko in which "-pre" and "-post" stash variables for
precomputed values were swapped.  Also accommodate :type string
variant for option `erc-fill-wrap-merge-indicator'.
(erc-fill--wrap-rejigger-region): Clear pre-computed merge-indicator
value.  (Bug#60936)