Vuepress interpolated anchors

[salazarr-js]

• I confirm that this is an issue rather than a question.

Bug report

Markdown title anchors don't work well with vue text interpolation.
When using {{ }} vue text interpolation in titles or headings, the markdown.anchors plugin doesn't generate correct slugs.

Steps to reproduce

https://codesandbox.io/s/vuepress-interpolated-anchors-j34bb?file=/README.md

1. Add some vue data variables in a script or in the frontmatter block

---
title: Vuepress interpolated anchors
---

<script>
export default {
  data: function() {
    return {
      subTitle: 'Interpolated headings examples'
    }
  }
}
</script>

2. Use them in some headings with vue text interpolation

# {{ $frontmatter.title }}

## {{ subTitle }}

What is expected?

We can expect that the anchors are the slug version of the content of the variables,

<h1 id="vuepress-interpolated-anchors">
  <a href="#vuepress-interpolated-anchors">#</a>
  ...
</h1>

<h2 id="interpolated-headings-examples">
  <a href="#interpolated-headings-examples">#</a>
  ...
</h2>

What is actually happening?

The anchors are the slug version of the variable name, no of its content

<h1 id="frontmatter-title">
  <a href="#frontmatter-title">#</a>
  ...
</h1>

<h2 id="subtitle">
  <a href="#subtitle">#</a>
  ...
</h2>

Other relevant information

Environment Info:
  System:
    OS: macOS 10.15.3
    CPU: (4) x64 Intel(R) Core(TM) i5-5350U CPU @ 1.80GHz
  Binaries:
    Node: 12.16.1 - /usr/local/opt/node@12/bin/node
    Yarn: Not Found
    npm: 6.13.4 - /usr/local/opt/node@12/bin/npm
  Browsers:
    Chrome: 81.0.4044.138
    Firefox: Not Found
    Safari: 13.0.5
  npmPackages:
    @vuepress/core:  1.4.1
    @vuepress/theme-default:  1.4.1
    vuepress: ^1.4.1 => 1.4.1
  npmGlobalPackages:
    vuepress: 1.4.1