Design Tokens: Add elevation tokens (shadows)

# Context

While elevation is a combination of multiple visual cues (background color, drop shadow, material..., etc.) that can even vary across Light and Dark modes, this issue focuses solely on defining a set of normalized design tokens for drop shadows rather than tackling all the possible combinations for conveying elevation.

# Platform

All.

```[tasklist]
# Design phase
- [ ] Tokens are added to Tokens Studio
- [ ] Tokens are published to Compound Styles Figma library
- [ ] Tokens are made available to Themer if necessary
- [ ] Tokens shareout for designers
```

```[tasklist]
# Development phase
- [ ] Tokens are generated for the target platforms
```