Preserve hard linebreaks for markdown files in pre-commit hook

[shiyingtu]

• Background: Currently, pre-commit trailing whitespace hook trims all whitespace from the ends of lines, which may change how the page is formatted from Markdown file. For example, hard linebreaks - preceded by two or more spaces will not be preserved

• Fix: add an arg called [--markdown-linebreak-ext=md] to preserve hard line break

[rgil-splunk]

also the dependency failure seems unrelated to this change and should be fixed by Peter's PR earlier today if you update your branch with the latest main