(fix) only provide extension version of prettier plugin if there is no workspace version (#492)

Only provide our version of the svelte plugin if the user doesn't have one in the workspace already. If we did it, Prettier would - for some reason - use the workspace version for parsing and the extension version for printing, which could crash if the contract of the parser output changed.
(which it did from 1.1.0 to 1.1.1)

Author: dummdidumm

packages/language-server/src/plugins/svelte/SveltePlugin.ts

@@ -69,7 +69,7 @@ export class SveltePlugin
             (await prettier.resolveConfig(filePath, { editorconfig: true })) || this.prettierConfig;
         const formattedCode = prettier.format(document.getText(), {
             ...config,
-            plugins: [require.resolve('prettier-plugin-svelte')],
+            plugins: getSveltePlugin(),
             parser: 'svelte' as any,
         });
 
@@ -79,6 +79,17 @@ export class SveltePlugin
                 formattedCode,
             ),
         ];
+
+        function getSveltePlugin() {
+            // Only provide our version of the svelte plugin if the user doesn't have one in
+            // the workspace already. If we did it, Prettier would - for some reason - use
+            // the workspace version for parsing and the extension version for printing,
+            // which could crash if the contract of the parser output changed.
+            const hasPluginLoadedAlready = prettier
+                .getSupportInfo()
+                .languages.some((l) => l.name === 'svelte');
+            return hasPluginLoadedAlready ? [] : [require.resolve('prettier-plugin-svelte')];
+        }
     }
 
     async getCompletions(document: Document, position: Position): Promise<CompletionList | null> {