support for `svelte.config.ts`

[sacrosanctic]

sveltejs/kit#13935 was added back in June, but the CLI still only checks for svelte.config.js.

cli/packages/cli/commands/add/utils.ts

Line 112 in 3edce89

svelteConfig: 'svelte.config.js',

[manuel3108]

We should detect the correct file (extension) and pass that to the addons similar to what we are doing with viteConfig