fix: support experimental.useVitePreprocess option for Vite 2.8 (#240)

Author: bluwy

.changeset/soft-masks-fold.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+fix `experimental.useVitePreprocess` option for Vite 2.8

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

@@ -25,7 +25,8 @@ function createViteScriptPreprocessor(): Preprocessor {
 			tsconfigRaw: {
 				compilerOptions: {
 					// svelte typescript needs this flag to work with type imports
-					importsNotUsedAsValues: 'preserve'
+					importsNotUsedAsValues: 'preserve',
+					preserveValueImports: true
 				}
 			}
 		});