Add configuration for overriding hardcoded icon settings

[Amai-ICE]

PR Description

Added a patcher to the function in pkg/gui/gui.go thet loads icons, allowing VCS and file icons to be customized via Lazygit's config instead of being hardcoded.

This PR is related to the following issue: #4549 #3863

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc