fix(types): specify CompleOptions.generate values (sveltejs#5321)

Author: benmccann

src/compiler/interfaces.ts

@@ -108,7 +108,7 @@ export interface CompileOptions {
 	format?: ModuleFormat;
 	name?: string;
 	filename?: string;
-	generate?: string | false;
+	generate?: 'dom' | 'ssr' | false;
 
 	outputFilename?: string;
 	cssOutputFilename?: string;
@@ -166,4 +166,4 @@ export interface Var {
 export interface CssResult { 
 	code: string;
 	map: SourceMap;
-}
+}