bug#60936: 30.0.50; ERC >5.5: Add erc-fill style based on visual-line-mode

[J.P.]

v3 (erc-display-line redux). Properly offset renarrowed region after
inserting initial date stamp in `erc-insert-timestamp-left-and-right'.
Don't displace third-party markers when inserting left-sided stamps in
`erc-stamp--display-margin-mode'.

The first bug was introduced by

  c68dc7786fc * Manage some text props for ERC insertion-hook members

and causes right-sided stamps to appear inside the prompt, among other
unpleasant things (see third patch). Thanks to Corwin for spotting this.
The other bug has been around a bit longer, likely since

  63d8b2a59a4 * Make erc-fill-wrap work with left-sided stamps

It has the potential to break packages that place markers in
modification hooks (see last patch).

0000-v2-v3.diff
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0001-Mark-erc-log-test-as-unstable.patch
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0002-5.6-Restore-missing-metadata-props-in-erc-display-li.patch
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0003-5.6-Fix-right-stamps-commingling-with-erc-prompt.patch
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0004-5.6-Respect-user-markers-in-erc-insert-timestamp-lef.patch
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