Add useTheme composable function

[innocenzi]

Add a useTheme composable function, usable with the Vue Composition API, that will allow to keep track of the currently used theme, set a theme, or manually get the current theme, for an element.

The theme.config.js would be given to the use function (or detected), or some options would be passable to it.

The current theme on an element would be detected according to the detected strategy, the default theme in the configuration, and the current prefers-color-scheme.

A theme would be changeable, the use function would detected the strategy and update it accordingly.

[innocenzi]

See #22