Don't inherit properties when refreshing ERC's prompt

* lisp/erc/erc.el (erc--merge-prop-behind-p): New variable to be
dynamically bound around rare calls to `erc--merge-props' when the
latter should append to the end of existing list-valued text
properties.
(erc--inhibit-prompt-display-property-p): New variable to be non-nil
in buffers where an active module needs to reserve all uses of the
`display' text property in the prompt region for itself.
(erc--prompt-properties): Collect all common prompt properties in one
place for code reuse and maintenance purposes.
(erc--refresh-prompt-continue, erc--refresh-prompt-continue-request):
New function and state variable for custom `erc-prompt' functions to
indicate to ERC that they need the prompt to be refreshed in all
buffers and not just the current one.
(erc--refresh-prompt): Merge `erc-prompt-face' behind any applied by a
customized `erc-prompt' function value.  Crucially, don't inherit
properties at the beginning of the prompt because doing so may clobber
any added by a custom `erc-prompt' function.  Instead, apply known
properties from `erc-display-prompt' manually.  Integrate
`erc--refresh-prompt-continue' logic.
(erc--merge-prop): Recognize flag to activate `append' behavior in
which new prop values are appended to the tail of existing ones rather
than consed in front.  This functionality could be extended to
arbitrary splices as well.
(erc-display-prompt): Use common text properties defined elsewhere.
* test/lisp/erc/erc-tests.el (erc--merge-prop): Add assertion for
`erc--merge-prop-behind-p' non-nil behavior.  (Bug#51082)

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