Optionally allow substitution patterns in erc-prompt

Optionally allow substitution patterns in erc-prompt

* etc/ERC-NEWS: Add entry for `erc-prompt-format'.
* lisp/erc/erc-compat.el (erc-compat--defer-format-spec-in-buffer):
New macro to wrap `format-spec' specification values in functions that
run in the current buffer and fall back to the empty string.
* lisp/erc/erc.el (erc-prompt): Add predefined Custom choice for
function type in `erc-prompt-format'.
(erc--prompt-format-face-example): New "pre-propertized" value for
option `erc-prompt-format'.
(erc-prompt-format): New companion option for `erc-prompt' choice
`erc-prompt-format'.  New function of the same name to perform format
substitutions and serve as a Custom choice value for `erc-prompt'.
Based on work and ideas originally proposed by Stefan Kangas.
(erc--away-indicator, erc-away-status-indicator,
erc--format-away-indicator): New formatting function and helper
variables for displaying short away status.
(erc--user-modes-indicator): New variable.
(erc--format-user-modes): New function.
(erc--format-channel-status-prefix): New function.
(erc--format-modes): New function.
* test/lisp/erc/erc-scenarios-prompt-format.el: New file.  (Bug#51082)

Co-authored-by: Stefan Kangas <stefankangas@gmail.com>