fix(markdown): decode assets link to ensure bundler can find the file

[Mister-Hope]

This is an issue in all VuePress versions.

![中国](./中国.png)

should be allowed. But markdown-it 's default behavior will encode it to ./%E4%B8%AD%E5%9B%BD.png, so that bundler won't be able to find the image file.

I made a fix in V1 by my vuepress-plugin-md-enhance, but I think V2 can support it.

[meteorlxy]

I just added more test cases, which will fail for now.

I think we should make all those cases passed.

(You can still modify those test cases as needed)

[Mister-Hope]

I don't think alias is needed, as I am not thinking of any situation which a person should define a alia containing chinese. While a markdown folder likely to contain chinese images,(like harmony os official docs), so relative links is necessary.

Even if we resolve those alias begin with @, alias are not forced to begin with @.

The only way to fit this is to decode back every link not begin with / , comparing with decoding every relative links now.

[meteorlxy]

I think just decode the original src is enough. No need to add too many codes

[meteorlxy]

Made some refactor. Thanks for your contribution ❤️