closes #6660

Previously, when formatting comments containing blockquotes followed by comparison operators like > >= >> >>= >>> >>>=, the code would panic with byte index out of bounds error.

The issue was in itemized_block_quote_start function, which counted all > characters in non-alphanumeric prefix, treating operators like >= as nested blockquote markers. This caused the calculated indent to exceed the line length, leading to out-of-bounds indexing.

Fixed by only recognizing valid Markdown blockquote syntax where each > must be followed by a space (e.g., > > text), which was the original intended behavior. Note that only the standard format with spaces is supported, alternative formats like >> text, > > text (multiple spaces), or >text (no space) are not recognized as blockquote markers.