Add cutoffs parameter to nice_fit() for controlling fit index cutoffs display

[Copilot]

This PR addresses JOSS review feedback regarding the nice_fit() function's handling of fit index cutoffs and documentation accuracy.

Key Changes

Added cutoffs parameter for explicit control over cutoff display:

# Hide cutoffs while keeping table formatting (new functionality)
nice_fit(model, nice_table = TRUE, cutoffs = FALSE)

# Show cutoffs (default behavior - unchanged)  
nice_fit(model, nice_table = TRUE, cutoffs = TRUE)

# Override guidelines setting
nice_fit(model, nice_table = TRUE, guidelines = TRUE, cutoffs = FALSE)

Updated terminology from "Common guidelines" to "Suggested cutoffs" in the table footer to better reflect the nature of these values.

Enhanced documentation with:

• Comprehensive @param cutoffs description explaining the new parameter
• Fixed grammar in Jorgensen quote reference (removed extra comma)
• Added missing Jorgensen et al. (2018) citation to @references section
• Updated warning text to mention both guidelines and cutoffs parameters

Maintained full backward compatibility - the cutoffs parameter defaults to the same value as guidelines, so existing code behavior is unchanged.

Validation

• Added comprehensive test coverage for the new cutoffs parameter functionality
• All existing tests continue to pass (45 tests total)
• Package builds and installs successfully
• Documentation generates correctly

Impact

This change gives users fine-grained control over cutoff display while addressing the JOSS review concerns about cutoff misuse. Users can now hide potentially misleading cutoffs while retaining the nice table formatting, which aligns with best practices for structural equation modeling.

The implementation respects the existing Jorgensen quote warning in the documentation while providing practical flexibility for users who understand the limitations of cutoffs but still find them useful as rough guidelines.

Fixes #21.

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