Add erc--spkr text property to chat messages

Add erc--spkr text property to chat messages

* etc/ERC-NEWS: Mention combined face ordering for "/me" messages.
* lisp/erc/erc-backend.el (erc-server-PRIVMSG): Bind
`erc--msg-prop-overrides' for the extent of this function's execution,
which means a large amount of code will see this variable as being
non-nil.  However, no calls to `erc-display-message' or
`erc-display-msg' should occur other than the one handling the final
insertion.  Code needing to influence the current message's "msg
props" can push new pairs onto this value, which will override any
existing collisions.
* lisp/erc/erc-fill.el (erc-fill): Switch to `erc--spkr' as sentinel
property.
(erc-fill--wrap-continued-message-p): Look for `erc--spkr' property
instead of `erc-speaker'.
* lisp/erc/erc.el (erc--msg-props): Mention `erc--spkr' in doc.
(erc--send-action-perform-ctcp): Add `erc--spkr' property.  Fix bug in
which `erc-my-nick-face' appeared below `erc-input-face' in the
speaker portion.
(erc--ensure-spkr-prop): New helper function to propagate speaker
metadata.
(erc--ranked-properties): Add `erc--spkr', `erc--ctcp', and
`erc--ephemeral'.
(erc-display-message): Use default hash table size when initializing.
Remove unnecessary assignment of `msg' to `erc--msg' for PRIVMSG and
NOTICE commands.  Bind `string' below `erc--msg-props' so that
implementers of the function form of the `erc-format-message'
templating interface can read and write the current context's "msg
props".
(erc--own-property-names): Add all `erc--msg-props' props by
subsumation.
(erc--get-speaker-bounds): Use `erc--spkr' instead of `erc--msg' as a
sentinel to detect a chat message guaranteed to have an `erc--speaker'
text-property interval.
(erc-format-privmessage, erc-format-my-nick, erc-ctcp-query-ACTION):
Add `erc--spkr' to `erc--msg-prop-overrides' when available.
* test/lisp/erc/erc-fill-tests.el: (erc-fill-tests--insert-privmsg):
Bind `erc--msg-prop-overrides'.
(erc-fill-tests--compare): Require environment variable value to match
current test name exactly when saving snapshots.  Add `erc--msg-props'
individually to white list.
* test/lisp/erc/erc-tests.el (erc--order-text-properties-from-hash):
Include `erc--spkr'.  (Bug#60936)  (Bug#67677)
; * test/lisp/erc/resources/fill/snapshots/merge-01-start.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/merge-02-right.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/merge-wrap-01.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-post-01.eld:
; Update.
; * test/lisp/erc/resources/fill/snapshots/merge-wrap-indicator-pre-01.eld:
; Update.
; * test/lisp/erc/resources/fill/snapshots/monospace-01-start.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/monospace-02-right.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/monospace-03-left.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/monospace-04-reset.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/spacing-01-mono.eld: Update.
; * test/lisp/erc/resources/fill/snapshots/stamps-left-01.eld: Update.

(cherry picked from commit 236a416be76cbb0b79ad46c06652f6cbf8788fcb)