fix(rehype): run postprocess transformers after code highlighting

[Subham-KRLX]

Fixes the postprocess transformer hook not being called when using @shikijs/rehype.

The issue occurred because @shikijs/rehype uses codeToHast() which returns a HAST tree, but postprocess transformers expect HTML strings. This PR bridges the gap by:

Detecting when postprocess transformers are present
Converting HAST → HTML using toHtml()
Running all postprocess transformer hooks
Parsing HTML back to HAST using fromHtml()
Linked Issues
Fixes #884

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.91%. Comparing base (5068b26) to head (6e8dcb6).
⚠️ Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1112      +/-   ##
==========================================
+ Coverage   88.35%   94.91%   +6.56%     
==========================================
  Files          74       90      +16     
  Lines        3322     7828    +4506     
  Branches      956     1641     +685     
==========================================
+ Hits         2935     7430    +4495     
- Misses        344      392      +48     
+ Partials       43        6      -37     

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[antfu]

Emm, I am not sure if this is the efficient way to do it, it's already be noted that postprocess will only be ran in certain conditions:

shiki/packages/types/src/transformers.ts

Lines 96 to 99 in 5068b26

	/**
	* Transform the generated HTML string before returning.
	* This hook will only be called with `codeToHtml`.
	*/

I think maybe instead, we could update the docs of rehype to mention about it, and tell users to use to-html from from-html within the root hook to swap the entire tree if they really want html string operation

[Subham-KRLX]

I have updated the PR to follow your guidance:

Reverted the postprocess hook execution in rehype

Added inline documentation explaining why postprocess only runs with codeToHtml

Provided example code showing how users can achieve HTML-based postprocessing using a root transformer with toHtml/fromHtml

Removed the postprocess test and unused dependencies

[antfu]

Shouldn't we do the docs in the documentation markdown instead? And the PR still contains unrelated changes;

[Subham-KRLX]

@antfu Both concerns addressed in the latest commit. Documentation moved to docs/packages/rehype.md
and unrelated changes removed.