perf(post): simplify codeblock escape

[SukkaW]

What does it do?

Since #4171 is about to be discarded, I picked up some changes into this PR.

This PR improves the performance of code blocks escape.

Before this PR, backtick_code_filter will output code block like this <escape>highlighted code</escape>, which will become <!--\uFFc${index}--> after PostRenderCache.escapeContent().

This PR removes PostRenderCache.escapeContent() by using <!--hexoPostRenderEscape: & :hexoPostRenderEscape-->. Why it would work please refer to #4171 (comment).

How to test

git clone -b post-render-backtick-code-filter https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

This PR improve generation speed by 3% without breaking current test cases.

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage decreased (-0.0006%) to 97.741% when pulling eef7d73 on SukkaW:post-render-backtick-code-escape into c51b87e on hexojs:master.

[jerryc127]

It shows the ':hexoPostRenderEscape–>' in the post, it cause by this commit ?