paste, join: support attached '-d=' / '-t=' single-'=' separators

[MadeNavaneeth]

Problem

While fixing #14120 (sort -t=), a systematic check of other utilities with single-character separator flags found the same clap limitation still unhandled in two more places:

command	expected (GNU)	actual before
paste -d= f1 f2	a=x / b=y	ax / by — delimiter silently lost
join -t= f1 f2	merged on =	empty output — no rows matched

Clap strips the first = after a short option, so the separator arrived as an empty string. cut already works around this for -d= (#2424), and #14144 does it for sort -t=.

Fix

• paste: rewrite attached -d<chars> to --delimiters=<chars> before parsing
• join: split attached -t<chars> into -t <chars> (join's -t has no long form; the separate form passes through verbatim). Caret diagnostics keep echoing the arguments as typed, per the guidance in clap_localization.rs.

Both rewrites preserve separators containing further = characters, and separate-form usage (paste -d =, join -t =) is untouched.

Testing

• paste: new equals-delim-attached case in the EXAMPLE_DATA table (1=2, a=b)
• join: new test_separator_attached_equals asserting GNU output byte-exact (1=a=b, 2=b=c)
• Full suite: 4702 passed; the only failure (test_human_numeric_blank_thousands_sep_locale) fails identically on clean master in this environment
• clippy and rustfmt clean for both crates

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/join/join. tests/join/join is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will not alter performance

✅ 10 untouched benchmarks
⏩ 398 skipped benchmarks¹

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_{Comparing MadeNavaneeth:fix/paste-join-equals-separator (a63a42e) with main (1a6fb19)²}

Footnotes

1. 398 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (7d10e5b) during the generation of this report, so 1a6fb19 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩