Add comprehensive autocomplete and IntelliSense support for BigQuery

[yossi55]

Summary

This PR adds comprehensive autocomplete and IntelliSense support to the SQLTools BigQuery driver, significantly improving the developer experience when writing BigQuery SQL queries.

✨ New Features

🎯 Static Completions

• BigQuery SQL Keywords: All major SQL keywords with proper prioritization
• 100+ BigQuery Functions: Comprehensive function library including:
  • Aggregate functions (ARRAY_AGG, COUNT, SUM, etc.)
  • Date/Time functions (DATE_ADD, PARSE_TIMESTAMP, etc.)
  • String functions (REGEXP_EXTRACT, CONCAT, etc.)
  • Math functions (ROUND, SQRT, etc.)
  • JSON functions (JSON_EXTRACT, JSON_VALUE, etc.)
  • Geography functions (ST_DISTANCE, ST_GEOGPOINT, etc.)
• Common Column Names: Frequently used column names for better WHERE clause support

🔍 Dynamic Completions

• Real-time BigQuery Metadata: Fetches actual projects, datasets, tables, and columns
• Intelligent Caching: 5-minute TTL for performance
• Fully Qualified Names: Support for project.dataset.table.column format
• Context-Aware Suggestions: Shows relevant metadata based on current context

🚀 Enhanced Search

• Comprehensive searchItems: Support for all BigQuery object types:
  • Databases (projects)
  • Schemas (datasets)
  • Tables and Views
  • Columns with data type information
  • Functions and Procedures
• Improved Column Search:
  • Better filtering by project/dataset
  • Data type icons for visual recognition
  • Support for partial and fully qualified names
  • Enhanced ordering to prioritize exact matches

🛠 Technical Improvements

• Enhanced getStaticCompletions method with prioritized suggestions
• New search queries for databases, schemas, views, and functions
• Improved searchItems method with better context handling
• Dynamic completion caching for performance optimization
• Better error handling for graceful degradation

📈 User Experience

Before

• Limited autocomplete support
• No function suggestions
• No metadata-based completions

After

• Ctrl+Space shows relevant suggestions in all contexts
• Intelligent prioritization (columns → keywords → functions)
• Rich documentation for all functions
• Real-time database metadata integration

🧪 Testing

Tested with:

• SELECT clause completions ✅
• Table and column suggestions ✅
• Function autocomplete with documentation ✅
• Project/dataset navigation ✅
• Performance with large schemas ✅

📝 Note

WHERE clause autocomplete has some limitations due to SQLTools core architecture (see related issues #110 and #371). This is a known limitation affecting all SQLTools drivers.

🎯 Impact

This significantly improves the BigQuery development experience in VSCode, making it much easier to:

• Discover available functions and their usage
• Navigate complex BigQuery schemas
• Write queries with confidence
• Reduce syntax errors and typos

Closes any existing issues related to autocomplete/IntelliSense functionality.