feat: add custom decoration color setting for StreamGuard extension

[Wiibleyde]

This pull request introduces a new feature to the StreamGuard extension that allows users to customize the color of the masking overlay via a new configuration setting. The implementation updates the configuration schema, extension logic, and documentation to support and explain this new option.

Custom Decoration Color Feature:

• Added a new configuration setting streamGuard.decorationColor to allow users to specify a custom CSS hex color for the masking overlay. If left empty, the extension falls back to the editor warning theme color. [1] [2] [3]
• Updated the decoration logic to use the user-specified color, including proper handling of color changes and caching of decoration types.
• Extended the configuration reading and propagation throughout the extension to ensure the custom color is passed to all relevant decoration and masking functions. [1] [2] [3] [4] [5] [6]

Documentation Updates:

• Updated README.md to document the new streamGuard.decorationColor setting, including example usage and a description in the configuration table. [1] [2]
• Added a changelog entry for version 1.3.0 describing the new custom decoration color feature.