Design Tokens: Add underlined text tokens

# Context

We need to use underlined text to indicate links or text buttons. In Figma, the underline property comes bundled with the core typography properties (font family, size, weight...) used for Text Styles.

As a result, if designers need to apply an underline to an existing Text Style, they'll need to be detach the style first, leaving the text layer disconnected from the text style. To prevent this from happening we need to introduce a set of underline text tokens. It should focus on the styles used for body and small copy, rather than those used for headings.

# 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
```