feature(starship): character config option

[schlich]

AI GENERATED SUMMARY

This pull request updates the Starship devcontainer feature to allow customizable prompt symbols and improves configuration handling. The most important changes are grouped below.

Feature enhancements and configuration:

• Added a new character option to the starship feature, enabling users to customize the prompt's success and error symbols via the devcontainer configuration (src/starship/devcontainer-feature.json).
• Updated the installation script to generate a starship.toml configuration file with the specified character settings, ensuring the prompt symbols are set during installation (src/starship/install.sh).

Testing improvements:

• Added a test scenario to validate changing prompt symbols through the new configuration option (test/starship/scenarios.json).
• Created a test script to verify that the success and error symbols in the prompt are correctly modified (test/starship/change_prompt_symbol.sh).