Implement code autocomplete and IntelliSense for DQL queries

Currently the plugin provides highlighting only. Adding autocomplete would significantly improve the user experience when writing DQL queries, similar to IDE support.

**Proposed Changes:**
- Show autocomplete suggestions for DQL commands, functions, and keywords as the user types
- Context-aware suggestions based on cursor position (e.g., suggest functions after `|` pipe)
- Support autocomplete for field names if connected to Dynatrace data schema
- Display function signatures and documentation on hover or autocomplete selection
- Support tab-completion for common patterns

**Benefits:**
- Faster query composition with less manual typing
- Discovery of available functions and commands
- Reduced syntax errors through guided input
- Better parity with Dynatrace's own query editor

**Related Code:**
The plugin would need to integrate with Obsidian's editor API. The lexer in `src/dql-highlight.ts` provides the foundation for building context-aware suggestions.