Fix issue #43: Export lexical variable getter and setter blocks from core

[SurbhiAgarwal1]

Summary

Fixes #43

This PR exports the lexical_variable_get and lexical_variable_set block definitions from the core module, making them available to developers who import the plugin.

Tasks Completed:

• Export lexical_variable_get block definition from variable-get-set.js
• Export lexical_variable_set block definition from variable-get-set.js
• Import blocks in core.js to ensure automatic registration
• Add blocks as static properties on LexicalVariablesPlugin class
• Re-export blocks from main index.js entry point
• Add idempotent registration guard (prevents duplicate registration)
• Update README.md with usage documentation
• Verify all tests pass (4/4 VariableGetSet tests passing)
• Build succeeds without errors

Why This Change?

The core fields (FieldGlobalFlydown and FieldParameterFlydown) create XML that references these blocks. Previously, when developers imported only the core module, these blocks weren't available, causing issues. Now they're automatically registered and accessible.

Testing

• ✅ All 4 VariableGetSet tests pass
• ✅ Build succeeds without errors
• ✅ No new test failures introduced
• ✅ Blocks are registered idempotently (safe to import multiple times)

Usage Examples

// Blocks are automatically registered when importing
import {LexicalVariablesPlugin} from '@mit-app-inventor/blockly-block-lexical-variables';

// Access block definitions directly
import {lexical_variable_get, lexical_variable_set} from '@mit-app-inventor/blockly-block-lexical-variables';

// Or from the core module
import {LexicalVariablesPlugin} from '@mit-app-inventor/blockly-block-lexical-variables/core';
const getterBlock = LexicalVariablesPlugin.lexical_variable_get;
const setterBlock = LexicalVariablesPlugin.lexical_variable_set;