Auto completion for transition directive

[yhatt]

By the latest Marp CLI update, #408 brings transition directive support. The exported HTML can play transition animations if the browser was supproted View Transition API (e.g. Chrome 111+).

We are going to add IntelliSense support of transition local directive with auto completion of built-in transitions.

As same as the theme, it's better that the author can see the animation of every built-in transitions in the pop-up help.

I've created marp-team/marp-cli-transitions-gif-generator for generating GIF animations for every transitions, and used in the documentation of Marp CLI transitions. Marp for VS Code is going to refer them too.