feat: add transformerNotationFold

[SHUBHI-KUMARI]

Description

This PR adds a new transformer transformerNotationFold to @shikijs/transformers. This transformer allows users to mark sections of code as foldable using comment notation, similar to VS Code or Monaco Editor.

It supports two syntaxes:

1. Shiki-style: // [!code fold:start] and // [!code fold:end]
2. Region-style: // #region and // #endregion

Changes

• Added packages/transformers/src/transformers/notation-fold.ts
• Exported transformerNotationFold in packages/transformers/src/index.ts
• Added tests in packages/transformers/test/fold.test.ts

Usage

Shiki Syntax

// [!code fold:start] variables
let a = 1
// [!code fold:end]

Region Syntax

// #region variables
let a = 1
// #endregion

Implementation Details

The transformer adds the following classes to the generated HTML:

• fold-start: Added to the line containing the start marker (for #region) or the line following the start marker (for [!code fold:start]).
• fold-end: Added to the line containing the end marker (for #endregion) or the line preceding the end marker (for [!code fold:end]).
• fold-content: Added to all lines between the start and end lines.

Note: Shiki-style markers ([!code fold:...]) are removed from the output, while #region markers are preserved (but still get the classes).

Verification

Added a new test file packages/transformers/test/fold.test.ts which verifies:

• Correct class application for both syntaxes.
• Nested folding support (via stack implementation).
• Correct handling of line removal for Shiki markers.

closes: #861

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.27%. Comparing base (28dd090) to head (39c2cbe).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1219      +/-   ##
==========================================
+ Coverage   95.20%   95.27%   +0.06%     
==========================================
  Files          92       93       +1     
  Lines        7922     8034     +112     
  Branches     1689     1708      +19     
==========================================
+ Hits         7542     7654     +112     
  Misses        374      374              
  Partials        6        6              

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