[Enhancement] Add Excel (.xlsx) export functionality to `save_table()`

[brianmsm]

User Story

As a researcher, I often need to share my results tables with colleagues who use Excel, or I want to use Excel for further formatting or creating custom plots. I want save_table() to support exporting directly to a .xlsx file.

Acceptance Criteria

• Enhance the existing save_table() function.
• The function should be able to detect the desired output from the file extension (e.g., path = "my_table.xlsx").
• It must be able to export any table generated by psymetrics (from compare_model_fit, model_estimates, etc.).
• The implementation should use a lightweight package without heavy dependencies (e.g., openxlsx is a good candidate).

Example Usage

fit_table <- compare_model_fit(fit_1, fit_2)

# Desired outcome
save_table(fit_table, path = "model_comparison.xlsx")