Expose insertion time as text prop in erc-stamp

Expose insertion time as text prop in erc-stamp

* lisp/erc/erc-stamp.el (erc-add-timestamp): Add new text property
`erc-timestamp' to store lisp time object formerly ensconced in a
closure.  Instead of creating a new lambda for the cursor-sensor
function of each message in a buffer, leave a gap between messages to
trip the sensor function.  The motivation behind this change is to
allow third parties access to valuable timestamp data already stored
by ERC anyway.  Of secondary importance is discouraging the reliance
on those lambdas as a means of detecting message bounds.  The gap now
serves a similar purpose.  Basically, the final character in a
message, a newline, will not have a timestamp or a sensor function.
In the rare instance the stamps module isn't loaded, the new
`erc-command' property can be used for this purpose instead.  Also,
instead of looking for the `invisible' text property at point, which
is normally `point-max' and thus outside the accessible portion of the
buffer, look at the beginning of the inserted message.  This allows
hook members running before this function to opt out of timestamps by
marking a message as invisible.
(erc-echo-timestamp): Make interactive and show timestamps even when
the variable `erc-echo-timestamps' is nil.
(erc--echo-ts-csf): Add new function to serve as value of
cursor-sensor function text properties.
* test/lisp/erc/erc-stamp-tests.el: New file.  (Bug#60936.)