Fix Accumulating Cell Merge Info While formatWorkbook

[vst]

This PR closes #142.

formatWorkbook accumulates cell merge information (_formattingMerges) along with style information as it iterates over a list of worksheet data.

Indeed, this is necessary for formatCell to process a single worksheet and to conclude with a list of cell ranges for merged cells.

However, once we are done with processing a single worksheet, we should reset the accumulated merge information to an empty list so that the next worksheet does not contain cell merges accumulated from previous worksheets' data.