Make header_style a Property of ExcelFormatter

[WillAyd]

Right now the header style of excel exports is defined as global to the pandas/io/formats/excel.py module:

pandas/pandas/io/formats/excel.py

Line 37 in 1a12c41

header_style = {"font": {"bold": True},

Because this exists outside of the ExcelFormatter class where it is used it is way more difficult to override the default than it needs to be. If this was moved from global scope to instance scope it would allow users to theoretically subclass the ExcelFormatter and easily define their own style for the header

This could also eliminate the need for any work on #19386