fix(enhanced-img): preserve ambient context for `*?enhanced` imports

[BastiDood]

Preserving the ambient context

#12224 changes the default export of *?enhanced modules from a value of type string into a value of type Picture (from vite-imagetools). However, this change caused the ambient.d.ts file to no longer be an ambient module. According to the TypeScript docs, top-level imports and exports automatically upgrade the ambient context into a module context. That means the declare module '*?enhanced' is no longer applied "ambiently", but now requires an explicit import somehow. This breaks svelte-check and other static analysis tooling because TypeScript would report that any module imported as *?enhanced have no existing type declarations.

The fix is quite simple: simply move the top-level import into the declare module syntax as shown in 4455725535f2ecf617029b83cbc6322c2fe677f2. This preserves the "ambient-ness" of the ambient.d.ts.

Ensuring that dependencies are present with pnpm

This PR also fixes a possible footgun when importing modules with pnpm. By default, pnpm does not hoist dependencies to the top level of the node_modules. The direct dependencies of each package is thus locally scoped to that package only. If a package does not declare the dependency, pnpm will not resolve it.

This is problematic for the types/ of @sveltejs/enhanced-img because it imports from vite and svelte. Previously, these were declared as devDependencies, which causes pnpm to prune them from module resolution.

This PR fixes this by upgrading vite and svelte as direct dependencies. Alternatively, this may be downgraded to peerDependencies instead. Let me know if this is preferable.

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