fix: use the resolved vite root (#247)

* fix: use the resolved vite root

Although vite-plugin-svelte needs to infer a root in order to pre-scan
svelte dependencies, it should use the root returned in configResolved
to ensure compatibility with backend integrations and other plugins that
might modify the root.

Since vite-plugin-svelte uses `enforce: 'pre'`, it's not viable for
other plugins to workaround this by using `enforce: 'pre'`, since the
behavior would become order-dependent on how the user applies the
plugins, which is error-prone and would result in a bad user experience.

* chore: add changeset

.changeset/popular-tigers-camp.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+use the resolved vite root to support backend integrations

packages/vite-plugin-svelte/src/utils/options.ts

@@ -112,6 +112,7 @@ export function resolveOptions(
 			...defaultOptions.compilerOptions,
 			...preResolveOptions.compilerOptions
 		},
+		root: viteConfig.root,
 		isProduction: viteConfig.isProduction
 	};
 	addExtraPreprocessors(merged, viteConfig);