Migrate deprecated config `transformMode` to `viteEnvironment`

[shuuji3]

> pnpm test:ci

...

 The Vitest environment nuxt defines the "transformMode". This options was deprecated in Vitest 4 and will be removed in the next major version. Please, use "viteEnvironment" instead.
...

[shuuji3]

ref. nuxt/test-utils#1482