Make header_style a Property of ExcelFormatter
Right now the header style of excel exports is defined as global to the `pandas/io/formats/excel.py` module:

https://github.com/pandas-dev/pandas/blob/1a12c41d201f56439510e683fadfed1218ea9067/pandas/io/formats/excel.py#L37

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