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.

Start with src/dql-highlight.ts to understand the existing lexer, then read Obsidian’s editor API documentation for autocomplete integration. Define how suggestions should vary by cursor context and whether Dynatrace schema data is available. Done means DQL commands, functions, keywords, and supported field names can be suggested, with signatures or documentation shown where applicable.

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