chore: rename 'seoTitle' to 'metaTitle'

Author: ulivz

Diff for: packages/@vuepress/core/lib/prepare/ClientComputedMixin.js

@@ -63,9 +63,9 @@ module.exports = siteData => {
 
     get $title () {
       const page = this.$page
-      const { seoTitle } = this.$page.frontmatter
-      if (typeof seoTitle === 'string') {
-        return seoTitle
+      const { metaTitle } = this.$page.frontmatter
+      if (typeof metaTitle === 'string') {
+        return metaTitle
       }
 
       const siteTitle = this.$siteTitle

Diff for: packages/docs/docs/theme/option-api.md

@@ -1,5 +1,5 @@
 ---
-seoTitle: Option API | Theme
+metaTitle: Option API | Theme
 ---
 
 # Option API

Diff for: packages/docs/docs/zh/plugin/option-api.md

@@ -1,5 +1,5 @@
 ---
-seoTitle: Option API | Plugin
+metaTitle: Option API | Plugin
 ---
 
 # Option API

Diff for: packages/docs/docs/zh/theme/option-api.md

@@ -1,5 +1,5 @@
 ---
-seoTitle: Option API | Theme
+metaTitle: Option API | Theme
 ---
 
 # Option API