The order of palette categories can already be set via editorTheme.palette.categories in the settings file.

This PR allows that to also be set via a theme plugin. This is a continuation of #5500

RED.plugins.registerPlugin('test-theme-plugin', {
        type: 'node-red-theme',
        // scripts: [],
        css: [
            'theme.css',
        ],
        palette: {
            theme: [ ... ]
            categories: ['network', 'function', 'subflow', 'common']
        }
    }) 

If not set, the default order is: ['subflows', 'common', 'function', 'network', 'sequence', 'parser', 'storage'] - with any unknown categories being appended to the palette in the order they are loaded.