s/// replacement: implement the \U \L \u \l \E case-conversion escapes

[sylvestre]

GNU sed supports case-conversion escapes in the replacement text of s///. None of them work in uutils sed today.

$ echo "abc def" | sed 's/b\(.*\)/\U&/'

script	uutils sed	GNU sed 4.9
s/b\(.*\)/\U&/	aUbc def	aBC DEF
s/b\(.*\)/\u&/	aubc def	aBc def
s/b\(.*\)/\L&/	a\Lbc def	abc def
s/b\(.*\)/\l&/	a\lbc def	abc def
s/b\(.*\)/\E&/	a\Ebc def	abc def

[dedsec-terminal]

Opened #557 for this — implements GNU \\U \L \u \l \E\ in \s///\ replacement (literals, &, \\1..9, with \g-per-occurrence reset and empty-match carry). Verified against GNU sed 4.9 on the issue table plus edge cases; \cargo test --lib\ 368 passed, integration 265 passed, clippy clean. PTAL.