Install in Idea via Settings → Plugins → Browse repositories → (search for) Svelte
Or download it from here and install it manually: https://plugins.jetbrains.com/plugin/10021-svelte
The plugin requires Intellij Ultimate for development, since it depends on the Javascript plugin.
To start the plugin from Intellij, execute the runIde task from the list of intellij gradle tasks. It downloads a 2016.2 runtime and executes that version of intellij with the svelte plugin included.
Run the buildPlugin task. The distribution will be created in build/distributions. Upload to Plugin Repository.