Model configurability naming changes

[dlqqq]

• Standardize terminology for model providers:
  • *LmProvider: language model provider
  • *EmProvider: embedding model provider
• Rename GlobalConfig to ChatConfig
• Add a "glossary" section to contributor docs that covers all of the acronyms used throughout the project
• Add a section to contributor docs regarding terminology preferences:
  • The name of a class, all lowercase, should only refer to an instance of that class. So for example, a variable/attribute named provider should always refer to an instance of *Provider, not the class or a provider ID.
  • Always label IDs by suffixing with _id.
  • Always label classes by suffixing with _class.