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 88.35%. Comparing base (5068b26) to head (b0c9ae7).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1112   +/-   ##
=======================================
  Coverage   88.35%   88.35%           
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  Files          74       74           
  Lines        3322     3322           
  Branches      956      954    -2     
=======================================
  Hits         2935     2935           
  Misses        344      344           
  Partials       43       43           

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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