Use overlay instead of text prop to hide ERC's prompt

Use overlay instead of text prop to hide ERC's prompt

* lisp/erc/erc-backend.el (erc--hidden-prompt-overlay):
New variable, a buffer-local handle for the prompt overlay.
(erc--reveal-prompt): Delete overlay instead of text prop.
(erc--conceal-prompt): Add overlay instead of text prop.
(erc--unhide-prompt): Run `erc--refresh-prompt-hook' after revealing.
(erc--hide-prompt): Run `erc--refresh-prompt-hook' after hiding.
* lisp/erc/erc-stamp.el (erc-stamp--adjust-margin): Attempt a more
accurate estimate of the prompt's width in columns when initially
setting left-margin.
(erc-stamp--skip-left-margin-prompt-p): New variable to inhibit normal
behavior of displaying prompt in left margin.
(erc-stamp--display-margin-mode): Allow opting out of
prompt-in-left-margin behavior.
(erc--reveal-prompt): Delete unneeded method implementation.
(erc--conceal-prompt): Put overlay in margin.
* test/lisp/erc/erc-tests.el (erc-hide-prompt): Use
`get-char-property' instead of `get-text-property' in order to
accommodate overlay-based prompt hiding.  (Bug#51082)