Comments on the rendered view of Markdown attachments store row numbers. I'm going to be changing the row layout to be block-based, and comments made on the old layout will need to remap to the matching rows of the new one. This change adds two new helpers for this: * `get_row_text_ranges()` returns where each row's text sits in the whole document. Whitespace isn't counted, so different layouts of the same Markdown line up. Images and horizontal rules count as one character, and each line of code gets one extra character so empty lines still have a position. * `map_text_ranges()` maps each row of one layout to the rows of another that its text overlaps. This handles rows that were merged (lines of a paragraph) and rows that were split (rows of a table). Rows without text map to the row before them. Nothing uses these yet. Testing Done: Ran unit tests.