[feature] Fix indent in imported code snippets

[xbill82]

What does this PR do?

Automatically fix the indentation of imported code snippets (useful when importing partial snippet files).

How should this be manually tested?

import a code-block with significant (consistent) indentation from a snippet file, like

function() {
  foo.forEach(f => {
    /* snippet:start */
    console.log(f)
    /* snippet:end */
  })
}

See how the imported snippet is

console.log(f) // Look ma! No spaces at the beginning of the line!

Other changes

Switched to vuepress-frontmatter-lint package.