echo: don't expand \u, \U and \" in -e

[SichenLiang]

uutils' echo -e expands \u, \U and \". GNU's echo has none of the three and
prints them as written. Our own --help already lists only the sequences GNU has:

\ \a \b \c \e \f \n \r \t \v \0NNN \xHH

so the implementation is the outlier here, not the documentation.

echo -e '[\u0041]'        GNU [\u0041]        before [A]    after [\u0041]
echo -e '[\U00000041]'    GNU [\U00000041]    before [A]    after [\U00000041]
echo -e '[\"]'            GNU [\"]            before ["]    after [\"]

Malformed forms were losing text as well, since parse_unicode consumes its digit
window before validating it:

echo -e '[\uzzzz]'        GNU [\uzzzz]        before [\x]   after [\uzzzz]

printf keeps all three, on its format string and on %b, and is byte identical
before and after across the escapes I tested, exit status and stderr included.

Measured against GNU coreutils 9.11, LC_ALL=C.

This touches escape.rs, mod.rs and echo.rs, which #14406 also edits. A trial merge
gives five mechanical conflict hunks and no semantic disagreement, so whichever lands
first the other is a small re-resolve.

Fixes #14414

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/link-heap is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!

[cakebaker]

Thanks!