Prose Gate Reads a Data Heredoc After a Shift by a Name as Code #3664

The prose gate reads a data heredoc in a workflow run: step as code when a shift by a name comes earlier in the same step. So a # line the heredoc writes into a file is reported as a comment, comment-added included.

Constructed input, x.yml:

- run: |
    (( y = x << n ))
    cat > f <<'EOF'
    # Data the step writes into the file.
    EOF

The gate reports line 4 as a comment line. It should read it as data, the way it reads the same heredoc with no shift before it.

The cause is in split_heredocs. The patched bash lexer reads << n as a heredoc opener that never closes. Since such an opener is more often a shift, the rest of the step stays one code piece. That piece is lexed by bash-heredoc-as-code, which has no heredoc rule, so any later heredoc in it is never split out.

develop before the #1664 fix behaves the same way, since it read every run-block heredoc body as code. Found by the local review pass on the #1664 pull request.