Fix strict TypeScript errors

[FloEdelmann]

app.component only allows string as a first parameter: https://github.com/vuejs/core/blob/422ef34e487f801e1162bed80c0e88e868576e1d/packages/runtime-core/src/apiCreateApp.ts#L44

But Splide.name and SplideSlide.name are typed as string | undefined: https://github.com/vuejs/core/blob/422ef34e487f801e1162bed80c0e88e868576e1d/packages/runtime-core/src/componentOptions.ts#L131

So to pass strict TypeScript type-checking, a non-null assertion operator can manually declare that the names are indeed non-nullable (see e.g. a3657c7/src/js/components/Splide/Splide.vue#L26).