keep trailing tab before a soft line break

[zayeem06]

parse_inline strips trailing whitespace from a text run that ends at a line ending with cmark_chunk_rtrim, which also removes tabs. The spec removes only trailing spaces at the end of a line before a soft or hard line break, so a trailing tab should survive: foo\t\nbar renders as <p>foo\nbar</p> here but <p>foo\t\nbar</p> in commonmark.js 0.31.2 and per the spec (section "Soft line breaks": "Spaces at the end of the line ... are removed"). Strip only trailing spaces at that site; the paragraph-final trim and the ATX trailing-hashtag trim, which the spec does strip tabs from, are left alone.

This fix was found and prepared with AI assistance and reviewed by me before submitting.