This PR refactors MultiQC's colour lightening functionality to use alpha transparency instead of mixing colours with white. This change ensures colours render correctly on both light and dark backgrounds.

• Updated mqc_colour_scale.lighten_colour() to return rgba strings with calculated alpha values instead of mixing with white
• Modified get_colour() method to use the new alpha-based lightening approach
• Updated bargraph plotting code to handle rgba color strings properly
• Added rgba color validation support in ValidatedConfig.parse_color()
• Refactored xenome module to use the common color lightening function instead of duplicating logic
• Updated somalier module's _make_col_alpha() to handle rgba input colors

⚠️ This is a fairly major PR that changes a key aspect of report rendering for all MultiQC runs