Optionally align prompt to prefix in erc-fill-wrap

Optionally align prompt to prefix in erc-fill-wrap

* lisp/erc/erc-fill.el (erc-fill-wrap-align-prompt): New option for
aligning prompt with leading portion of messages at the common "static
center" pivot-column barrier, so it appears "dedented" along with all
speaker name tags.  Tests for this functionality appear in the
subsequent patch of this same change set.
(erc-fill-wrap-use-pixels): Demote from user option to normal variable
because it has no practical use other than for testing.  Don't rename
as internal variable to spare the improbable user of ERC on HEAD who's
already customized this.
(erc-fill-wrap-mode, erc-fill-wrap-enable, erc-fill-wrap-disable):
Take care to disable prompt-in-left-margin behavior when option
`erc-fill-wrap-align-prompt' is non-nil.
(erc-fill--wrap-measure): Improve doc string and always attempt to
leverage `buffer-text-pixel-size', even when the variable
`erc-fill-wrap-use-pixels' is nil.
(erc-fill--wrap-indent-prompt): New function to massage prompt
`line-prefix' after updates, such as changes to away status.
(Bug#51082)