Configurable text line height in Label

### What is the problem or limitation you are having?

The Label accepts multiple lines, but there's no way to control their spacing.

### Describe the solution you'd like

A `line_height` style with the same meaning as in CSS. See #2144 for a specification.

### Describe alternatives you've considered

The user can split their text into multiple Labels, but because they can't be overlapped, there will be a certain minimum spacing that they can't go below.

### Additional context

Compared to Canvas (#2144), the Label version of this feature would be more work to implement, but probably useful to more people.

Start by reading the line-height specification in issue #2144, then locate the Label implementation and its existing tests in the Toga repository. The work is complete when Label accepts a line_height style with the CSS-defined meaning and the behavior is covered across the relevant GUI backends.

Written by the indexing model from the issue text.