Draft: Initial merged cells support in `get_cells()`

[tzakharko]

Initial implementation for #289

This adds new columns n_rows and n_cols to the result of get_cells()/range_read_cells(). API response with the merged cell ranges is parsed in get_merged_ranges() and then left-joined by starting location into the cell table.

No updates to documentation, tests, or range formatting yet. Could also use error handling when implementing the join.