There were too many patches to the Render system, it's really difficult
to make further improvements.
This PR clears the legacy problems and fix TODOs.
1. Rename `RenderContext.Type` to `RenderContext.MarkupType` to clarify
its usage.
2. Use `ContentMode` to replace `meta["mode"]` and `IsWiki`, to clarify
the rendering behaviors.
3. Use "wiki" mode instead of "mode=gfm + wiki=true"
4. Merge `renderByType` and `renderByFile`
5. Add more comments
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The problem of "mode=document": in many cases it is not set, so many
non-comment places use comment's hard line break incorrectly
To make the markup package easier to maintain:
1. Split some go files into small files
2. Use a shared util.NopCloser, remove duplicate code
3. Remove unused functions
