ENH: Improve Excel header formatting with CSS-like font weight support

[antznette1]

@rhshadrach

Key Changes

1. Font Weight Support:

   • Added support for CSS-like font-weight properties in Excel exports
   • Enhanced font weight conversion for both xlsxwriter and openpyxl engines
   • Improved style inheritance and merging

2. New Parameter:

   • Added header_bold parameter to to_excel() for easy header formatting
   • Maintained backward compatibility with existing code

3. Code Quality:

   • Improved type hints
   • Enhanced documentation
   • Added comprehensive tests

Usage Examples

# Using CSS-like font-weight
styler = df.style.set_properties(**{"font-weight": "bold"})

# Using direct properties
styler = df.style.set_properties(font_weight="bold")

# Using to_excel parameter
df.to_excel("output.xlsx", header_bold=True)

### Related Issues
Part of the improvements from #62670

[rhshadrach]

It seems like this is just about the entire diff from #62670, not just the changes formatting.