Add Lexicon Parser and Code Generator

[tobbbles]

Implements a complete system for parsing ATProto lexicon JSON specifications and generating well-documented Elixir modules.

Features:

• Lexicon.Parser: Parses lexicon JSON with strict validation per ATProto spec

  • Supports query and procedure types
  • Extracts parameters with constraints (min/max, format, enum, etc.)
  • Validates NSID format and lexicon version

• Lexicon.Generator: Generates Elixir modules from parsed lexicons

  • Creates comprehensive module and function documentation
  • Generates proper @SPEC annotations
  • Generates correct HTTP implementations (GET for queries, POST for procedures)
  • Formats output code using Code.format_string!/1

• Mix Task: mix exosphere.gen.lexicon

  • Igniter-based task for interactive code generation
  • Supports --dry-run for previewing changes
  • Custom lexicon and output directories

Test Coverage:

• 79 tests covering parser, generator, and integration scenarios
• Test fixtures with realistic lexicon examples