fix(parser): honor backslash-newline line continuations in here-documents

[cataggar]

Second defect found while running an 88k-line bash CLI (kenji-cli) under brush; surfaced as a diff in one subcommand's --help output.

Root cause

In a here-document whose delimiter is unquoted, bash removes a backslash-newline pair outright and joins the two lines. A backslash is only literal there when it precedes $, a backtick, or another backslash.

heredoc_escape_sequence covered those three characters but not newline, so the pair fell through to heredoc_literal_text and both the backslash and the newline were emitted verbatim.

$ cat <<EOF
> alpha \
> beta
> EOF

	output
bash	alpha beta
brush (before)	alpha \ / beta

Fix

Add an explicit line-continuation rule and exclude it from the literal text rule. A quoted delimiter suppresses expansion entirely, so that path is untouched and continues to preserve the backslash — matching bash.

Testing

• 4 new compat cases: unquoted continuation, quoted-delimiter (backslash preserved), continuation before an expansion, and a combined case exercising \$ / \` / \\ / continuation together
• All 43 here-doc/here-string compat cases pass against the bash 5.2.15 oracle
• brush-parser: 224/224 unit tests pass
• cargo clippy and cargo fmt --check clean