more robust MessageFormat syntax

In ICU 4.8 we improved on the MessageFormat syntax, so that ASCII apostrophe is usually treated as literal text rather than starting quoted literal text. However, we still see problems. In particular, some translators type pairs of ASCII apostrophes rather than ASCII double-quote characters, and they expect the pairs to show as such; and some translators enclose arguments/placeholders in ASCII apostrophes or end a sub-message with single-quoted text, which still breaks the syntax.

See the "Problems with old syntax" in the new-syntax design doc.

Design new, more robust, less collision-prone syntax and allow for old and new syntax to coexist.