feat: add config customization support

[desmondinho]

Closes #3

Continue to expand the functionality of this package according to the original one.
For this PR I've added the config customization support that allows:

• Set LRU cache maximum capacity
• Set custom prefix (e.g., "tw-"; tw-block)
• Set custom separator (e.g., "_"; hover_bg-red-500)

Override and/or Extend

• Theme
• Class groups
• Conflicting class groups
• Conflicting class group modifiers

.. and all this can be done using AddTailwindMerge extension method that accepts a delegate of Action<TwMergeConfig>:

builder.Services.AddTailwindMerge(options =>
{
    options.CacheSize = 100;
    options.Prefix = "tw-";
    options.Separator = "_";

    options.Override( new ExtendedConfig()
    {
        Theme = [],
        ClassGroups = [],
        ConflictingClassGroups = [],
        ConflictingClassGroupModifiers = []
    } );

    options.Extend( new ExtendedConfig()
    {
        Theme = [],
        ClassGroups = [],
        ConflictingClassGroups = [],
        ConflictingClassGroupModifiers = []
    } );
};