chore: add '@ui5/theming-ngx' dependency always

[ilhan007]

Prior to this PR the @ui5/theming-ngx package was missing from the projects dependencies.

Issue
Prior to this PR,

• If you get started from scratch with
  ng create my-app
• And if you install @ui5/webcomponents-ngx with
  npm install @ui5/webcomponents-ngx,
• And then use some of the components in the app and run the app
  ng serve
• you will get an error that an import to @ui5/theming-ngx could not be resolved
  and you need to manually install the @ui5/theming-ngx package in addition no matter you want or don't need theming

Most probably it was optimisation effort to exclude the package and only add the dependency if the app requires theming. However, it's not working fine. Even if we use ng add @ui5/webcomponents-ngx the user is not asked to install it and the result is the same - we will get an error on ng serve.

Solution
Add @ui5/theming-ngx as dependency unconditionally to avoid breaking on first steps of the usage.

Fixes: #62

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