fix: default sorter doesn't work fine in Safari

[billyyyyy3320]

Summary

Because only the dates in frontmatter written in 2-digits will be transformed, other dates written in single-digit, such as 2020-1-1 will be treated as string. Some browsers (e.g. Safari) don't support this format.

Relevant issue: vuejs/vuepress#2097

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

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The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

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• A convincing reason for adding this feature
• Related documents have been updated
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To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

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