Add flag to force cell value to be returned as raw string

[johnnyliang7]

This PR add ForceString flag to return raw cell values, this gives user an “opt-out” of all of the library’s automatic type-conversions and just hand you back the raw value from the cell XML

• ForceString bool: new field on XlsxFile (default false).
• SetForceString(force bool): method to toggle forcing raw strings.
• getCellValue change: if ForceString==true, always return *r.Value (no numeric/date/shared-string conversion).
• Tests added (TestGettingValueFromRawCell_ForceString): verifies raw output for numeric, date-style, shared-string, and inline-string cells when ForceString is enabled.