[Feature] Highlight matching closing bracket / symbol (parentheses, brackets, braces)

**Description**

Hi! First of all, thanks for maintaining AST Explorer — it’s an incredibly useful tool.

I’d like to propose a small UX feature that could greatly improve code readability and navigation.

**Feature request:**
Add support for highlighting the matching closing symbol when the cursor is placed on an opening one (and vice versa), for example:

Parentheses: ( )

Brackets: [ ]

Braces: { }

This behavior is common in many editors (VS Code, IntelliJ, etc.) and is especially helpful when working with deeply nested code or complex AST transformations.

**Expected behavior:**

When the cursor is on an opening symbol, highlight the corresponding closing symbol.

Optionally, also work in reverse (cursor on closing symbol highlights the opening one).

**Why this is useful:**

Makes it easier to understand code structure

Reduces mistakes when editing nested expressions

Improves overall developer experience when experimenting with ASTs

Thanks for considering this suggestion!