Add type definition for WorksheetModel.merges

[ytjmt]

Summary

Closes #1341

Test plan

I confirmed the type is Range['range'][]:

• exceljs/lib/doc/worksheet.js

  Lines 877 to 881 in ec92cb3

  	// Merges
  	model.merges = [];
  	_.each(this._merges, merge => {
  	model.merges.push(merge.range);
  	});

• exceljs/lib/doc/worksheet.js

  Lines 612 to 632 in ec92cb3

  	_mergeCellsInternal(dimensions, ignoreStyle) {
  	// check cells aren't already merged
  	_.each(this._merges, merge => {
  	if (merge.intersects(dimensions)) {
  	throw new Error('Cannot merge already merged cells');
  	}
  	});
  	// apply merge
  	const master = this.getCell(dimensions.top, dimensions.left);
  	for (let i = dimensions.top; i <= dimensions.bottom; i++) {
  	for (let j = dimensions.left; j <= dimensions.right; j++) {
  	// merge all but the master cell
  	if (i > dimensions.top || j > dimensions.left) {
  	this.getCell(i, j).merge(master, ignoreStyle);
  	}
  	}
  	}
  	// index merge
  	this._merges[master.address] = dimensions;

• exceljs/lib/doc/worksheet.js

  Lines 608 to 609 in ec92cb3

  	const dimensions = new Range(cells);
  	this._mergeCellsInternal(dimensions, true);

[Siemienik]

LGTM, thank you