Fix missing resources for en-US culture

[Copilot]

Problem

When running a website with Tag Manager installed, a warning appears in the Event Log:

Missing resources for culture en-US:
Kentico.Xperience.TagManager.Resources.TagManagerResources

This occurs because the library only registered localization resources for SystemContext.SYSTEM_CULTURE_NAME (which is "en"), but many Xperience installations run with the "en-US" culture. When Kentico's LocalizationDataProvider looks for resources in the en-US culture, it cannot find them and logs a warning.

Solution

This PR adds support for the en-US culture by:

1. Creating TagManagerResources.en-US.resx - A culture-specific resource file containing the same localization strings as the default resource file
2. Registering the en-US culture - Adding a second [assembly: RegisterLocalizationResource] attribute to inform Kentico's localization system that this resource supports en-US
3. Ensuring resource compilation - Explicitly including .resx files in the project to generate the en-US satellite assembly (required because the Razor SDK project has EnableDefaultEmbeddedResourceItems disabled)

Technical Details

The .NET build system compiles TagManagerResources.en-US.resx into a satellite assembly (Kentico.Xperience.TagManager.resources.dll) placed in an en-US subfolder. When the application runs with en-US culture, Kentico's localization system can now find and load the appropriate resources, eliminating the warning.

Changes

• Added src/Kentico.Xperience.TagManager/Resources/TagManagerResources.en-US.resx
• Updated src/Kentico.Xperience.TagManager/Resources/TagManagerResources.cs to register en-US culture
• Updated src/Kentico.Xperience.TagManager/Kentico.Xperience.TagManager.csproj to explicitly include resource files

Fixes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Missing resources for en-US</issue_title>
<issue_description>Describe the bug
When booting up a website with tag manager installed, you get the warning message:

Missing resources for culture en-US:
Kentico.Xperience.TagManager.Resources.TagManagerResources

To Reproduce
Steps to reproduce the behavior:

1. Launch website
2. Go to Event Log module in the admin site
3. See LocalizationDataProvider warning

Expected behavior
The warning should not appear.

Screenshots

Library Version
4.2.2</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #76

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