feat: transform vue template with posthtml

[egoist]

Summary

By transforming Vue template with PostHTML, Saber could add features for all types of pages at the same time.

For example, previously you can only use relative link to resolve page in markdown:

[About](./about.md)

This was implemented in a markdown plugin and only works for markdown page.

Now you can also use <saber-link>:

<saber-link to="./about.md">About</saber-link>

Or even using <a> directly:

<a href="./about.md">About</a>

<a> is converted to <saber-link> now if it's not an external link, so we will use <a> in docs and examples in the future.

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/CSS related code, please provide the before/after screenshot:

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

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

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Fixes #474 Fixes #460.

#472 can now be implemented as a PostHTML plugin instead.

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #460: Windows path issue in $saber.getPageLink

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