Added ThemeGenerator script for generating combined Fluent XAML theme files

[dipeshmsft]

Description

In this PR, I have added a simple PS script to combine all the Fluent theme XAML files into one. I have also removed some duplicate keys from the XAML files for this.

The ThemeGenerator.Fluent script unlike the previous script does not provide many functionalities ( like optimizing keys, and checking for DynamicResource references ) . It is a very simple script at the moment but depending on the necessities, we will keep adding features to this.

There is a reference to this task in : #8991

Customer Impact

Instead of loading multiple merged ResourceDictionaries, developer will load a single large ResourceDictionary which will help boost performance.

Regression

N/A

Testing

Yet to be done.

Risk

N/A

Microsoft Reviewers: Open in CodeFlow

[lindexi]

This reminds me of @batzen 's XAMLTools, which can combine the XAML.

[dipeshmsft]

This reminds me of @batzen 's XAMLTools, which can combine the XAML.

That is a much-advanced version of this script 😄

[dipeshmsft]

Closing this in favor of #9422