fix(bash): keep heredoc continuations highlighted

[puneetdixit200]

Fixes #4377.

Summary

• Keep Bash heredoc continuation commands on the command line instead of folding them into the heredoc string.
• Start the heredoc string span on the following line and end it at the matching delimiter.
• Add a Bash markup regression for <<-EOF | cat.
• Update the existing Bash heredoc expectation and changelog entry.

Tests

• Red before fix: the reported sample highlighted EOF | cat inside the heredoc string span, so cat was not highlighted as a command.
• ONLY_LANGUAGES=bash npm run test-markup
• npx eslint --no-eslintrc -c .eslintrc.lang.js src/languages/bash.js
• npm run build
• git diff --check

Note: I also ran the full npm run test-markup; it reached 537 passing tests and then hit the existing 2s Mocha timeout on http/default, unrelated to the Bash fixture. The Bash markup suite passed in both focused runs.