Use an asterisk to toggle emphasis (#478)

This matches `remark-stringify` defaults.

Author: remcohaszing

.changeset/empty-donkeys-notice.md

@@ -0,0 +1,7 @@
+---
+'@mdx-js/language-service': patch
+'@mdx-js/language-server': patch
+'vscode-mdx': patch
+---
+
+Use an asterisk to toggle emphasis

packages/language-server/test/syntax-toggle.test.js

@@ -80,14 +80,14 @@ test('emphasis', async () => {
       changes: {
         'memory://1': [
           {
-            newText: '_',
+            newText: '*',
             range: {
               end: {character: 0, line: 0},
               start: {character: 0, line: 0}
             }
           },
           {
-            newText: '_',
+            newText: '*',
             range: {
               end: {character: 5, line: 0},
               start: {character: 5, line: 0}

packages/language-service/lib/service-plugin.js

@@ -94,7 +94,7 @@ export function createMdxServicePlugin(options) {
                 context,
                 options,
                 'emphasis',
-                '_',
+                '*',
                 args[0],
                 args[1]
               )