fix: add more asset-searching locations and solve a known issue

[lucienshawls]

This resolves #216.

Until now, if I would like to insert an image named image.jpg in a post with the filename foo.md with these configurations:

post_asset_folder: true
relative_link: false
marked:
  prependRoot: true
  postAsset: true

and the filesystem structure:

source/_posts
├── foo.md
└── foo/
    └── image.jpg

I have to use ![](image.jpg) instead of ![](foo/image.jpg) otherwise hexo would not generate a valid img src. However, if I would like to preview the markdown file while editing, I have to use the latter one. This is contradictory.

So I made a modification to allow hexo-renderer-marked to search for assets not only in current asset folder source/_posts/foo, but also in its parent folder source/_posts, so that ![](foo/image.jpg) can also be located and the src can be generated correctly.

Moreover, this change is compatible with the previous versions and developers can safely update the package to a new version without the need for changing each and every link in their blogs.

Notice: the changes have passed the tests on Linux (WSL2, Ubuntu 22.04).

I hope these changes are acceptable.

[coveralls]

coverage: 100.0%. remained the same
when pulling a325590 on lucienshawls:master
into 1b09354 on hexojs:master.

[DexterMorgan0417]

the check is not passed ,your idea is very good,i have the same demand

[lucienshawls]

I managed to resolve the lint problems. See if it helps.

[bl-sun]

I need this.