Peek for closing backtick delimiter when inside a code span (is_literal != ""). Handles both single-backtick and double-backtick spans:
is_literal length 1 = LITERAL span, length 2 = CODE span. Only closes on an exact run of the opener length; longer runs are treated as
content (e.g. two or three backticks inside a one-backtick span). When in a single-backtick span and or ` is seen (with no
more chars or next not `), sets flush_chars to 2 or 3 so the caller can flush that many characters as content.
| flush_chars |
set to 2 or 3 when |
|
0 no match (treat as content), -1 need more data, N match length in bytes |