[Performance] Reduce repeated block-config scans in AST2BlockList._propertyOf

[Rohit-rk07]

Problem

AST2BlockList._propertyOf() repeatedly:

• scans config.body_blocks to resolve each block's metadata
• recomputes connection slot indices (prev, child, next, second_child) via repeated indexOf

During large AST-to-blockList conversions, this becomes avoidable overhead in a hot path.

Why It Matters

JS import/export conversion speed impacts responsiveness when handling larger projects.

Proposed Change

• Precompute a blockName -> config entry map once per toBlockList() call.
• Cache computed connection indices per connection-array object and reuse them.

Scope

• File: js/js-export/ast2blocklist.js
• Methods: _treeToBlockList() / _propertyOf()

Acceptance Criteria

• Output blockList structure and ordering remain unchanged.
• Fallback behavior for unknown blocks remains unchanged.
• Repeated linear scans/index lookups in _propertyOf() are removed or minimized via caching.
• Existing ast2blocklist tests pass.