Edit link can't be disabled via frontmatter

[dirkdev98]

Describe the bug

When edit link is enabled in the config, disabling it via frontmatter on a page is not possible.
This seems to be caused here https://github.com/vuejs/vitepress/blob/master/src/client/theme-default/composables/editLink.ts#L19.

Intending to open a PR if the expected behaviour is agreed upon.

Reproduction

• Set editLink to true in the config
• Add editLink: false to the frontmatter of a documentation page
• The Edit this page link still shows up

Expected behavior

Expecting that when either editLink: false or editLink: "false" is passed in via frontmatter, that the edit link is not rendered on that page.

System Info

System:
  OS: Linux 5.10 Manjaro Linux
  CPU: (8) x64 Intel(R) Core(TM) i7-8809G CPU @ 3.10GHz
  Memory: 19.23 GB / 31.28 GB
  Container: Yes
  Shell: 5.1.4 - /bin/bash
Binaries:
  Node: 16.0.0 - ~/.config/nvm/versions/node/v16.0.0/bin/node
  Yarn: 1.22.5 - ~/.yarn/bin/yarn
  npm: 7.10.0 - ~/.config/nvm/versions/node/v16.0.0/bin/npm
npmPackages:
  vitepress: 0.15.6 => 0.15.6

Additional context

No response

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