fix: should not pass entry files to MarkdownIt

[KermanX]

fix #1772.

This is a regression caused by #1767. In which the check of whether the processed file is the entry file was removed from the before hook of unplugin-vue-markdown.

And inside handleHotUpdate, await ctx.read was called because of #884. Unfortunately, unplugin-vue-markdown overrides this method here. This causes unexpected processing of the entry file, while the client hasn't import it at all.

This PR fixes this by adding order: 'pre' to the loader plugin to use await ctx.read before unplugin-vue-markdown, and re-adding the checks in the before hook of unplugin-vue-markdown.

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