fix($core): SFC page compile error (close: #1110)

This is an issue caused by TSC compilation.

Author: ulivz

packages/@vuepress/shared-utils/src/parseVueFrontmatter.ts

@@ -1,5 +1,4 @@
 // @ts-ignore
-import compiler from 'vue-template-compiler'
 import { parse as _parse } from '@vue/component-compiler-utils'
 import parseFrontmatter from './parseFrontmatter'
 
@@ -11,7 +10,7 @@ export function parseStrippedFrontmatter (src: string) {
 export function parse (src: string) {
   const output = _parse({
     source: src,
-    compiler,
+    compiler: require('vue-template-compiler'),
     needMap: false
   })
   const find = output.customBlocks.find(block => block.type === 'frontmatter')