meta tags aren't being escaped in build, producing broken HTML

[runofthemillgeek]

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

Bug report

Steps to reproduce

1. Create a new VuePress project with the default theme
2. Add frontmatter with description set to some HTML in a markdown file
3. Run yarn dev, notice no issues
4. Run yarn build and open the HTML page, notice broken HTML

I have a reproduction available here: https://github.com/sangeeth96/bug-vuepress-meta-description-repro

What is expected?

Meta tag contents are escaped before being added to HTML.

What is actually happening?

Meta tag contents aren't being escaped resulting in broken HTML.

Other relevant information

• Output of npx vuepress info in my VuePress project:

  Environment Info:
  
  System:
   OS: macOS 11.2.3
   CPU: (8) x64 Intel(R) Core(TM) i5-8279U CPU @ 2.40GHz
  Binaries:
   Node: 14.16.1 - ~/.nvm/versions/node/v14.16.1/bin/node
   Yarn: 1.22.10 - /usr/local/bin/yarn
   npm: 6.14.12 - ~/.nvm/versions/node/v14.16.1/bin/npm
  Browsers:
   Chrome: 90.0.4430.72
   Edge: Not Found
   Firefox: Not Found
   Safari: 14.0.3
  npmPackages:
   @vuepress/core:  1.8.2 
   @vuepress/theme-default:  1.8.2 
   vuepress: ^1.3.1 => 1.8.2 
  npmGlobalPackages:
   vuepress: Not Found
  

[runofthemillgeek]

I've got a PR to fix this #2832 (if this is all valid). Wasn't sure how to add test for this but I can try if someone could give me a hint.