[Refactor] Move all icons to common `StudyBuddyIcons` object

Part of the Compose migration #368, consider migrating all vector drawable-based icons to their Compose `ImageVector` equivalent.

We can use the [Valkyire IDE plugin](https://plugins.jetbrains.com/plugin/24786-valkyrie--svg-to-imagevector) for this, which can generate the following structure:

```kotlin
object StudyBuddyIcons {
object Filled
object Outlined
}

val StudyBuddyIcons.Filled.Help: ImageVector by lazy { /* ... */ }
```

## Tasks

* [x] Add `:ui:icons` (or `:core:designsystem:icons`)